Adopted Span and MutableSpan for buffers across the library to adapt to Swift 6.4.

Sources/PlaydateKit/Sound/Source/Aliases/SoundCallbackSource.swift

@@ -1,6 +1,6 @@
 extension Sound.CallbackSource {
     /// Fills the sample buffers and returns `true` if output was
-    /// produced. `right` is non-nil only for stereo sources.
-    public typealias Callback = (_ left: UnsafeMutableBufferPointer<Int16>,
-                                 _ right: UnsafeMutableBufferPointer<Int16>?) -> Bool
+    /// produced. `right` is empty for mono sources.
+    public typealias Callback = (_ left: inout MutableSpan<Int16>,
+                                 _ right: inout MutableSpan<Int16>) -> Bool
 }

Sources/PlaydateKit/Sound/Source/Classes/CallbackSource.swift

@@ -27,9 +27,9 @@ extension Sound {
                                                UnsafeMutablePointer<Int16>?, Int32) -> Int32 = { context, left, right, length in
             guard let context, let left else { return 0 }
             let source = Unmanaged<CallbackSource>.fromOpaque(context).takeUnretainedValue()
-            let leftBuffer = UnsafeMutableBufferPointer(start: left, count: Int(length))
-            let rightBuffer = right.map { UnsafeMutableBufferPointer(start: $0, count: Int(length)) }
-            return source.callback(leftBuffer, rightBuffer) ? 1 : 0
+            var leftSpan = UnsafeMutableBufferPointer(start: left, count: Int(length)).mutableSpan
+            var rightSpan = UnsafeMutableBufferPointer(start: right, count: right == nil ? 0 : Int(length)).mutableSpan
+            return source.callback(&leftSpan, &rightSpan) ? 1 : 0
         }
 
         /// Attaches the C object created for this source.

Sources/PlaydateKit/Sound/Source/Classes/FilePlayer.swift

@@ -7,7 +7,7 @@ extension Sound {
 
         var loopCallback: ((FilePlayer) -> Void)?
         var fadeCallback: ((FilePlayer) -> Void)?
-        var mp3DataSource: ((UnsafeMutableBufferPointer<UInt8>) -> Int)?
+        var mp3DataSource: ((inout MutableSpan<UInt8>) -> Int)?
         private var retainedRateModulator: SignalValue?
 
         override init(pointer: OpaquePointer?, isOwned: Bool) {
@@ -131,13 +131,13 @@ extension Sound {
         /// fills the buffer and returns the number of bytes written; return 0
         /// to signal the end of the stream.
         public func setMP3StreamSource(bufferLength: Float,
-                                       _ dataSource: @escaping (UnsafeMutableBufferPointer<UInt8>) -> Int) {
+                                       _ dataSource: @escaping (inout MutableSpan<UInt8>) -> Int) {
             mp3DataSource = dataSource
             FilePlayer.api.pointee.setMP3StreamSource.unsafelyUnwrapped(pointer, { data, bytes, userdata in
                 guard let userdata, let data else { return 0 }
                 let player = Unmanaged<FilePlayer>.fromOpaque(userdata).takeUnretainedValue()
-                let buffer = UnsafeMutableBufferPointer(start: data, count: Int(bytes))
-                return Int32(player.mp3DataSource?(buffer) ?? 0)
+                var buffer = UnsafeMutableBufferPointer(start: data, count: Int(bytes)).mutableSpan
+                return Int32(player.mp3DataSource?(&buffer) ?? 0)
             }, Unmanaged.passUnretained(self).toOpaque(), bufferLength)
         }