[Audio] Correct float PCM endpoint conversion (#291)

src/SharpEmu.Libs/Audio/AudioPcmConversion.cs

@@ -52,8 +52,19 @@ internal static class AudioPcmConversion
         }
 
         var bits = BinaryPrimitives.ReadInt32LittleEndian(sample);
-        var value = Math.Clamp(BitConverter.Int32BitsToSingle(bits), -1.0f, 1.0f);
-        return checked((short)MathF.Round(value * short.MaxValue));
+        return ConvertFloatSample(BitConverter.Int32BitsToSingle(bits));
+    }
+
+    private static short ConvertFloatSample(float value)
+    {
+        if (float.IsNaN(value))
+        {
+            return 0;
+        }
+
+        value = Math.Clamp(value, -1.0f, 1.0f);
+        var scale = value < 0.0f ? 32768.0f : short.MaxValue;
+        return checked((short)MathF.Round(value * scale));
     }
 
     private static short ApplyVolume(short sample, float volume)

tests/SharpEmu.Libs.Tests/Audio/AudioPcmConversionTests.cs

@@ -0,0 +1,57 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using System.Buffers.Binary;
+using SharpEmu.Libs.Audio;
+using Xunit;
+
+namespace SharpEmu.Libs.Tests.Audio;
+
+public sealed class AudioPcmConversionTests
+{
+    [Fact]
+    public void FloatFullScaleMapsToSignedPcmEndpoints()
+    {
+        Span<byte> source = stackalloc byte[sizeof(float) * 2];
+        WriteFloat(source, 0, -1.0f);
+        WriteFloat(source, 1, 1.0f);
+        Span<byte> destination = stackalloc byte[AudioPcmConversion.OutputFrameSize];
+
+        AudioPcmConversion.ConvertToStereoPcm16(
+            source,
+            destination,
+            frames: 1,
+            channels: 2,
+            bytesPerSample: sizeof(float),
+            isFloat: true,
+            volume: 1.0f);
+
+        Assert.Equal(short.MinValue, BinaryPrimitives.ReadInt16LittleEndian(destination));
+        Assert.Equal(short.MaxValue, BinaryPrimitives.ReadInt16LittleEndian(destination[2..]));
+    }
+
+    [Fact]
+    public void FloatNaNMapsToSilence()
+    {
+        Span<byte> source = stackalloc byte[sizeof(float)];
+        WriteFloat(source, 0, float.NaN);
+        Span<byte> destination = stackalloc byte[AudioPcmConversion.OutputFrameSize];
+
+        AudioPcmConversion.ConvertToStereoPcm16(
+            source,
+            destination,
+            frames: 1,
+            channels: 1,
+            bytesPerSample: sizeof(float),
+            isFloat: true,
+            volume: 1.0f);
+
+        Assert.Equal(0, BinaryPrimitives.ReadInt16LittleEndian(destination));
+        Assert.Equal(0, BinaryPrimitives.ReadInt16LittleEndian(destination[2..]));
+    }
+
+    private static void WriteFloat(Span<byte> destination, int sample, float value) =>
+        BinaryPrimitives.WriteInt32LittleEndian(
+            destination[(sample * sizeof(float))..],
+            BitConverter.SingleToInt32Bits(value));
+}