diff --git a/Sources/PlaydateKit/Sound/Effect/Classes/Effect.swift b/Sources/PlaydateKit/Sound/Effect/Classes/Effect.swift
index a76e784..7d1ec25 100644
--- a/Sources/PlaydateKit/Sound/Effect/Classes/Effect.swift
+++ b/Sources/PlaydateKit/Sound/Effect/Classes/Effect.swift
@@ -38,10 +38,16 @@ extension Sound {
         }
 
         deinit {
-            if isOwned {
-                effectAPI.pointee.freeEffect.unsafelyUnwrapped(pointer)
+            // Subclasses free the C object in their own deinit with the
+            // subsystem's type-specific free (freeDelayLine, freeOverdrive,
+            // ...); freeing here as well would double-free. The base class
+            // owns only the custom-processor effects it creates itself.
+            if let processorBox {
+                if isOwned {
+                    effectAPI.pointee.freeEffect.unsafelyUnwrapped(pointer)
+                }
+                processorBox.release()
             }
-            processorBox?.release()
         }
 
         /// The wet/dry mix: 1 is fully processed, 0 fully dry.
diff --git a/Tests/PlaydateKit/MockPlaydate.swift b/Tests/PlaydateKit/MockPlaydate.swift
index 50b1685..8d21f1a 100644
--- a/Tests/PlaydateKit/MockPlaydate.swift
+++ b/Tests/PlaydateKit/MockPlaydate.swift
@@ -20,6 +20,10 @@ enum Mock {
     nonisolated(unsafe) static let spriteAPI = UnsafeMutablePointer<playdate_sprite>.allocate(capacity: 1)
     nonisolated(unsafe) static let soundAPI = UnsafeMutablePointer<playdate_sound>.allocate(capacity: 1)
     nonisolated(unsafe) static let channelAPI = UnsafeMutablePointer<playdate_sound_channel>.allocate(capacity: 1)
+    nonisolated(unsafe) static let synthAPI = UnsafeMutablePointer<playdate_sound_synth>.allocate(capacity: 1)
+    nonisolated(unsafe) static let soundEffectAPI = UnsafeMutablePointer<playdate_sound_effect>.allocate(capacity: 1)
+    nonisolated(unsafe) static let lfoAPI = UnsafeMutablePointer<playdate_sound_lfo>.allocate(capacity: 1)
+    nonisolated(unsafe) static let delayLineAPI = UnsafeMutablePointer<playdate_sound_effect_delayline>.allocate(capacity: 1)
     nonisolated(unsafe) static let fileAPI = UnsafeMutablePointer<playdate_file>.allocate(capacity: 1)
     nonisolated(unsafe) static let jsonAPI = UnsafeMutablePointer<playdate_json>.allocate(capacity: 1)
     nonisolated(unsafe) static let apiStruct = UnsafeMutablePointer<PlaydateAPI>.allocate(capacity: 1)
@@ -39,6 +43,19 @@ enum Mock {
     nonisolated(unsafe) static var spriteUpdateCallback: (@convention(c) (OpaquePointer?) -> Void)?
     nonisolated(unsafe) static var audioCallback: (@convention(c) (UnsafeMutableRawPointer?, UnsafeMutablePointer<Int16>?, UnsafeMutablePointer<Int16>?, Int32) -> Int32)?
     nonisolated(unsafe) static var audioContext: UnsafeMutableRawPointer?
+    /// Generator registrations per synth. The OS deallocs a generator's
+    /// userdata when the generator is replaced or its synth is freed; the
+    /// mock mirrors that contract.
+    struct SynthGenerator {
+        var render: (@convention(c) (UnsafeMutableRawPointer?, UnsafeMutablePointer<Int32>?, UnsafeMutablePointer<Int32>?, Int32, UInt32, Int32) -> Int32)?
+        var dealloc: (@convention(c) (UnsafeMutableRawPointer?) -> Void)?
+        var userdata: UnsafeMutableRawPointer?
+    }
+    nonisolated(unsafe) static var synthGenerators: [OpaquePointer: SynthGenerator] = [:]
+    /// The last custom-effect proc handed to `newEffect`, plus each
+    /// effect's userdata, for re-invocation by tests.
+    nonisolated(unsafe) static var effectProc: (@convention(c) (OpaquePointer?, UnsafeMutablePointer<Int32>?, UnsafeMutablePointer<Int32>?, Int32, Int32) -> Int32)?
+    nonisolated(unsafe) static var effectUserdata: [OpaquePointer: UnsafeMutableRawPointer] = [:]
 
     static func record(_ event: String) {
         events.append(event)
@@ -72,6 +89,9 @@ enum Mock {
         spriteUpdateCallback = nil
         audioCallback = nil
         audioContext = nil
+        synthGenerators = [:]
+        effectProc = nil
+        effectUserdata = [:]
     }
 
     private static func install() {
@@ -282,6 +302,86 @@ enum Mock {
             Mock.record("channelRemoveSource")
             return 1
         }
+        channelAPI.pointee.addSource = { _, _ in
+            Mock.record("channelAddSource")
+            return 1
+        }
+
+        soundAPI.pointee.synth = UnsafePointer(synthAPI)
+        synthAPI.initialize(to: playdate_sound_synth())
+        synthAPI.pointee.newSynth = {
+            Mock.record("newSynth")
+            return Mock.fakePointer()
+        }
+        synthAPI.pointee.freeSynth = { synth in
+            Mock.record("freeSynth")
+            // Freeing a synth deallocs its generator's userdata.
+            if let synth, let generator = Mock.synthGenerators.removeValue(forKey: synth) {
+                generator.dealloc?(generator.userdata)
+            }
+        }
+        synthAPI.pointee.setGenerator = { synth, _, render, _, _, _, dealloc, _, userdata in
+            Mock.record("setGenerator")
+            guard let synth else { return }
+            // Replacing a generator deallocs the previous one first.
+            if let previous = Mock.synthGenerators[synth] {
+                previous.dealloc?(previous.userdata)
+            }
+            Mock.synthGenerators[synth] = SynthGenerator(render: render, dealloc: dealloc,
+                                                         userdata: userdata)
+        }
+        synthAPI.pointee.setFrequencyModulator = { _, _ in
+            Mock.record("setFrequencyModulator")
+        }
+
+        soundAPI.pointee.lfo = UnsafePointer(lfoAPI)
+        lfoAPI.initialize(to: playdate_sound_lfo())
+        lfoAPI.pointee.newLFO = { _ in
+            Mock.record("newLFO")
+            return Mock.fakePointer()
+        }
+        lfoAPI.pointee.freeLFO = { _ in
+            Mock.record("freeLFO")
+        }
+
+        soundAPI.pointee.effect = UnsafePointer(soundEffectAPI)
+        soundEffectAPI.initialize(to: playdate_sound_effect())
+        soundEffectAPI.pointee.newEffect = { proc, userdata in
+            Mock.record("newEffect")
+            let pointer = Mock.fakePointer()
+            Mock.effectProc = proc
+            if let userdata {
+                Mock.effectUserdata[pointer] = userdata
+            }
+            return pointer
+        }
+        soundEffectAPI.pointee.freeEffect = { effect in
+            Mock.record("freeEffect")
+            if let effect {
+                Mock.effectUserdata[effect] = nil
+            }
+        }
+        soundEffectAPI.pointee.getUserdata = { effect in
+            guard let effect else { return nil }
+            return Mock.effectUserdata[effect]
+        }
+
+        soundEffectAPI.pointee.delayline = UnsafePointer(delayLineAPI)
+        delayLineAPI.initialize(to: playdate_sound_effect_delayline())
+        delayLineAPI.pointee.newDelayLine = { _, _ in
+            Mock.record("newDelayLine")
+            return Mock.fakePointer()
+        }
+        delayLineAPI.pointee.freeDelayLine = { _ in
+            Mock.record("freeDelayLine")
+        }
+        delayLineAPI.pointee.addTap = { _, _ in
+            Mock.record("addTap")
+            return Mock.fakePointer()
+        }
+        delayLineAPI.pointee.freeTap = { _ in
+            Mock.record("freeTap")
+        }
     }
 
     // MARK: - File
diff --git a/Tests/PlaydateKit/WrapperTests.swift b/Tests/PlaydateKit/WrapperTests.swift
index 6c15e71..793772a 100644
--- a/Tests/PlaydateKit/WrapperTests.swift
+++ b/Tests/PlaydateKit/WrapperTests.swift
@@ -182,6 +182,148 @@ struct WrapperTests {
         #expect(Sound.CallbackSource.live.count == baseline)
     }
 
+    // MARK: Sound ownership graph
+
+    @Test func channelRetainsAddedSourceUntilRemoved() {
+        let channel = Sound.Channel()
+        weak var weakSynth: Sound.Synth?
+        do {
+            let synth = Sound.Synth()
+            weakSynth = synth
+            channel.addSource(synth)
+        }
+        #expect(weakSynth != nil)   // the channel keeps the source alive
+        #expect(Mock.eventCount("freeSynth") == 0)
+
+        if let synth = weakSynth {
+            channel.removeSource(synth)
+        }
+        #expect(weakSynth == nil)
+        #expect(Mock.eventCount("freeSynth") == 1)
+    }
+
+    @Test func synthRetainsItsModulatorWhileAlive() {
+        weak var weakLFO: Sound.LFO?
+        do {
+            let synth = Sound.Synth()
+            do {
+                let lfo = Sound.LFO()
+                weakLFO = lfo
+                synth.frequencyModulator = lfo
+            }
+            #expect(weakLFO != nil)   // the synth retains the modulator
+            #expect(Mock.eventCount("freeLFO") == 0)
+        }
+        #expect(weakLFO == nil)
+        #expect(Mock.eventCount("freeLFO") == 1)
+    }
+
+    @Test func synthGeneratorDispatchesAndIsReleasedWithTheSynth() {
+        final class Token {}
+        weak var weakToken: Token?
+        var rendered = 0
+
+        do {
+            let synth = Sound.Synth()
+            let token = Token()
+            weakToken = token
+            synth.setGenerator(stereo: false, .init(render: { left, _, _, _ in
+                _ = token
+                rendered += 1
+                return left.count
+            }))
+
+            // Simulate the audio engine rendering through the trampoline.
+            let generator = try! #require(Mock.synthGenerators[synth.pointer])
+            var samples = [Int32](repeating: 0, count: 8)
+            let frames = samples.withUnsafeMutableBufferPointer { buffer in
+                generator.render?(generator.userdata, buffer.baseAddress, nil,
+                                  Int32(buffer.count), 0, 0)
+            }
+            #expect(frames == 8)
+            #expect(rendered == 1)
+            #expect(weakToken != nil)
+        }
+        // freeSynth deallocs the generator userdata, releasing the box and
+        // the closure's captures with it.
+        #expect(Mock.eventCount("freeSynth") == 1)
+        #expect(weakToken == nil)
+    }
+
+    @Test func replacingASynthGeneratorReleasesThePreviousOne() {
+        final class Token {}
+        weak var firstToken: Token?
+
+        let synth = Sound.Synth()
+        do {
+            let token = Token()
+            firstToken = token
+            synth.setGenerator(stereo: false, .init(render: { left, _, _, _ in
+                _ = token
+                return left.count
+            }))
+        }
+        #expect(firstToken != nil)
+
+        synth.setGenerator(stereo: false, .init(render: { left, _, _, _ in left.count }))
+        #expect(firstToken == nil)   // the OS deallocs the replaced generator
+    }
+
+    @Test func effectProcessorDispatchesAndIsReleasedOnDeinit() {
+        final class Token {}
+        weak var weakToken: Token?
+        var processed = 0
+
+        do {
+            let token = Token()
+            weakToken = token
+            let effect = Sound.Effect(processor: { left, right, _ in
+                _ = token
+                processed += left.count
+                #expect(right == nil)
+                return true
+            })
+
+            // Drive the effect the way the OS would: through the registered
+            // proc, which recovers the box from the effect's userdata.
+            var samples = [Int32](repeating: 0, count: 4)
+            let active = samples.withUnsafeMutableBufferPointer { buffer in
+                Mock.effectProc?(effect.pointer, buffer.baseAddress, nil,
+                                 Int32(buffer.count), 1)
+            }
+            #expect(active == 1)
+            #expect(processed == 4)
+        }
+        #expect(Mock.eventCount("freeEffect") == 1)
+        #expect(weakToken == nil)
+    }
+
+    @Test func effectSubclassIsFreedExactlyOnceWithItsOwnFree() {
+        do {
+            let line = Sound.DelayLine(length: 256)
+            _ = line
+        }
+        #expect(Mock.eventCount("freeDelayLine") == 1)
+        // The base Effect deinit must not also free the subclass's object.
+        #expect(Mock.eventCount("freeEffect") == 0)
+    }
+
+    @Test func delayLineTapKeepsItsDelayLineAlive() {
+        weak var weakLine: Sound.DelayLine?
+        var tap: Sound.DelayLineTap?
+        do {
+            let line = Sound.DelayLine(length: 256)
+            weakLine = line
+            tap = line.addTap(delay: 128)
+        }
+        #expect(weakLine != nil)   // the tap retains its delay line
+        #expect(Mock.eventCount("freeDelayLine") == 0)
+
+        tap = nil
+        #expect(Mock.eventCount("freeTap") == 1)
+        #expect(Mock.eventCount("freeDelayLine") == 1)
+    }
+
     // MARK: File
 
     @Test func fileHandleReadsWritesAndClosesExactlyOnce() throws {