diff --git a/libraries/transformer/src/test/java/androidx/media3/transformer/FloatAudioMixingAlgorithmTest.java b/libraries/transformer/src/test/java/androidx/media3/transformer/FloatAudioMixingAlgorithmTest.java
index d5455f9226..bbc8311d37 100644
--- a/libraries/transformer/src/test/java/androidx/media3/transformer/FloatAudioMixingAlgorithmTest.java
+++ b/libraries/transformer/src/test/java/androidx/media3/transformer/FloatAudioMixingAlgorithmTest.java
@@ -18,6 +18,7 @@ package androidx.media3.transformer;
 import static androidx.media3.test.utils.TestUtil.createByteBuffer;
 import static androidx.media3.test.utils.TestUtil.createFloatArray;
 import static com.google.common.truth.Truth.assertThat;
+import static com.google.common.truth.Truth.assertWithMessage;
 
 import androidx.media3.common.C;
 import androidx.media3.common.audio.AudioProcessor.AudioFormat;
@@ -33,7 +34,6 @@ public final class FloatAudioMixingAlgorithmTest {
       new AudioFormat(/* sampleRate= */ 44100, /* channelCount= */ 2, C.ENCODING_PCM_FLOAT);
   private static final AudioFormat AUDIO_FORMAT_MONO_PCM_FLOAT =
       new AudioFormat(/* sampleRate= */ 44100, /* channelCount= */ 1, C.ENCODING_PCM_FLOAT);
-
   private static final AudioFormat AUDIO_FORMAT_STEREO_PCM_16BIT =
       new AudioFormat(/* sampleRate= */ 44100, /* channelCount= */ 2, C.ENCODING_PCM_16BIT);
   private static final AudioFormat AUDIO_FORMAT_MONO_PCM_16BIT =
@@ -45,6 +45,8 @@ public final class FloatAudioMixingAlgorithmTest {
       ChannelMixingMatrix.create(/* inputChannelCount= */ 1, /* outputChannelCount= */ 2);
   private static final ChannelMixingMatrix STEREO_TO_MONO =
       ChannelMixingMatrix.create(/* inputChannelCount= */ 2, /* outputChannelCount= */ 1);
+  private static final ChannelMixingMatrix MONO_TO_MONO =
+      ChannelMixingMatrix.create(/* inputChannelCount= */ 1, /* outputChannelCount= */ 1);
 
   @Test
   public void supportsSourceAudioFormatsForStereoMixing() {
@@ -76,8 +78,8 @@ public final class FloatAudioMixingAlgorithmTest {
         STEREO_TO_STEREO.scaleBy(0.5f),
         /* frameCount= */ 2,
         mixingBuffer);
-    assertThat(sourceBuffer.remaining()).isEqualTo(0);
-    assertThat(mixingBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Source buffer").that(sourceBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Mixing buffer").that(mixingBuffer.remaining()).isEqualTo(0);
 
     mixingBuffer.flip();
     assertThat(createFloatArray(mixingBuffer)).isEqualTo(new float[] {0f, -0.125f, 0.375f, -0.25f});
@@ -95,8 +97,8 @@ public final class FloatAudioMixingAlgorithmTest {
         MONO_TO_STEREO.scaleBy(0.5f),
         /* frameCount= */ 2,
         mixingBuffer);
-    assertThat(sourceBuffer.remaining()).isEqualTo(0);
-    assertThat(mixingBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Source buffer").that(sourceBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Mixing buffer").that(mixingBuffer.remaining()).isEqualTo(0);
 
     mixingBuffer.flip();
     assertThat(createFloatArray(mixingBuffer)).isEqualTo(new float[] {0f, -0.5f, 0.75f, -0.25f});
@@ -114,13 +116,32 @@ public final class FloatAudioMixingAlgorithmTest {
         STEREO_TO_MONO.scaleBy(0.5f),
         /* frameCount= */ 2,
         mixingBuffer);
-    assertThat(sourceBuffer.remaining()).isEqualTo(0);
-    assertThat(mixingBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Source buffer").that(sourceBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Mixing buffer").that(mixingBuffer.remaining()).isEqualTo(0);
 
     mixingBuffer.flip();
     assertThat(createFloatArray(mixingBuffer)).isEqualTo(new float[] {0.1875f, 0.5625f});
   }
 
+  @Test
+  public void mixMonoFloatIntoMonoFloat() {
+    AudioMixingAlgorithm algorithm = new FloatAudioMixingAlgorithm(AUDIO_FORMAT_MONO_PCM_FLOAT);
+
+    ByteBuffer mixingBuffer = createByteBuffer(new float[] {0.25f, -0.25f});
+    ByteBuffer sourceBuffer = createByteBuffer(new float[] {0.5f, 0.25f});
+    algorithm.mix(
+        sourceBuffer,
+        AUDIO_FORMAT_MONO_PCM_FLOAT,
+        MONO_TO_MONO.scaleBy(0.5f),
+        /* frameCount= */ 2,
+        mixingBuffer);
+    assertWithMessage("Source buffer").that(sourceBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Mixing buffer").that(mixingBuffer.remaining()).isEqualTo(0);
+
+    mixingBuffer.flip();
+    assertThat(createFloatArray(mixingBuffer)).isEqualTo(new float[] {0.5f, -0.125f});
+  }
+
   @Test
   public void mixStereoS16IntoStereoFloat() {
     AudioMixingAlgorithm algorithm = new FloatAudioMixingAlgorithm(AUDIO_FORMAT_STEREO_PCM_FLOAT);
@@ -140,8 +161,8 @@ public final class FloatAudioMixingAlgorithmTest {
         STEREO_TO_STEREO.scaleBy(0.5f),
         /* frameCount= */ 2,
         mixingBuffer);
-    assertThat(sourceBuffer.remaining()).isEqualTo(0);
-    assertThat(mixingBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Source buffer").that(sourceBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Mixing buffer").that(mixingBuffer.remaining()).isEqualTo(0);
 
     mixingBuffer.flip();
     assertThat(createFloatArray(mixingBuffer))
@@ -163,8 +184,8 @@ public final class FloatAudioMixingAlgorithmTest {
         MONO_TO_STEREO.scaleBy(0.5f),
         /* frameCount= */ 2,
         mixingBuffer);
-    assertThat(sourceBuffer.remaining()).isEqualTo(0);
-    assertThat(mixingBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Source buffer").that(sourceBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Mixing buffer").that(mixingBuffer.remaining()).isEqualTo(0);
 
     mixingBuffer.flip();
     assertThat(createFloatArray(mixingBuffer))
@@ -173,6 +194,59 @@ public final class FloatAudioMixingAlgorithmTest {
         .inOrder();
   }
 
+  @Test
+  public void mixStereoS16IntoMonoFloat() {
+    AudioMixingAlgorithm algorithm = new FloatAudioMixingAlgorithm(AUDIO_FORMAT_MONO_PCM_FLOAT);
+
+    ByteBuffer mixingBuffer = createByteBuffer(new float[] {0.25f, 0.5f});
+    ByteBuffer sourceBuffer =
+        createByteBuffer(
+            new short[] {
+              -16384 /* -0.5f */,
+              8192 /* 0.25000762962f */,
+              -8192 /* -0.25f */,
+              16384 /* 0.50001525925f */
+            });
+
+    algorithm.mix(
+        sourceBuffer,
+        AUDIO_FORMAT_STEREO_PCM_16BIT,
+        STEREO_TO_MONO.scaleBy(0.5f),
+        /* frameCount= */ 2,
+        mixingBuffer);
+    assertWithMessage("Source buffer").that(sourceBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Mixing buffer").that(mixingBuffer.remaining()).isEqualTo(0);
+
+    mixingBuffer.flip();
+    assertThat(createFloatArray(mixingBuffer))
+        .usingTolerance(1f / Short.MAX_VALUE)
+        .containsExactly(new float[] {0.1875f, 0.5625f})
+        .inOrder();
+  }
+
+  @Test
+  public void mixMonoS16IntoMonoFloat() {
+    AudioMixingAlgorithm algorithm = new FloatAudioMixingAlgorithm(AUDIO_FORMAT_MONO_PCM_FLOAT);
+
+    ByteBuffer mixingBuffer = createByteBuffer(new float[] {0.25f, 0.5f});
+    ByteBuffer sourceBuffer =
+        createByteBuffer(new short[] {-16384 /* -0.5f */, 8192 /* 0.25000762962f */});
+    algorithm.mix(
+        sourceBuffer,
+        AUDIO_FORMAT_MONO_PCM_16BIT,
+        MONO_TO_MONO.scaleBy(0.5f),
+        /* frameCount= */ 2,
+        mixingBuffer);
+    assertWithMessage("Source buffer").that(sourceBuffer.remaining()).isEqualTo(0);
+    assertWithMessage("Mixing buffer").that(mixingBuffer.remaining()).isEqualTo(0);
+
+    mixingBuffer.flip();
+    assertThat(createFloatArray(mixingBuffer))
+        .usingTolerance(1f / Short.MAX_VALUE)
+        .containsExactly(new float[] {0f, 0.625f})
+        .inOrder();
+  }
+
   @Test
   public void doesNotSupportSampleRateConversion() {
     AudioMixingAlgorithm algorithm =