diff --git a/demos/transformer/src/main/java/androidx/media3/demo/transformer/ConfigurationActivity.java b/demos/transformer/src/main/java/androidx/media3/demo/transformer/ConfigurationActivity.java
index bf731b6ab4..4fc52c001f 100644
--- a/demos/transformer/src/main/java/androidx/media3/demo/transformer/ConfigurationActivity.java
+++ b/demos/transformer/src/main/java/androidx/media3/demo/transformer/ConfigurationActivity.java
@@ -118,6 +118,7 @@ public final class ConfigurationActivity extends AppCompatActivity {
   public static final int HIGH_PITCHED_INDEX = 0;
   public static final int SAMPLE_RATE_INDEX = 1;
   public static final int SKIP_SILENCE_INDEX = 2;
+  public static final int CHANNEL_MIXING_INDEX = 3;
 
   // Color filter options.
   public static final int COLOR_FILTER_GRAYSCALE = 0;
@@ -164,7 +165,7 @@ public final class ConfigurationActivity extends AppCompatActivity {
     "720p H264 video with no audio",
   };
   private static final String[] AUDIO_EFFECTS = {
-    "High pitched", "Sample rate of 48000Hz", "Skip silence"
+    "High pitched", "Sample rate of 48000Hz", "Skip silence", "Mix channels into mono"
   };
   private static final String[] VIDEO_EFFECTS = {
     "Dizzy crop",
diff --git a/demos/transformer/src/main/java/androidx/media3/demo/transformer/TransformerActivity.java b/demos/transformer/src/main/java/androidx/media3/demo/transformer/TransformerActivity.java
index e5c3875e1c..4222ba1525 100644
--- a/demos/transformer/src/main/java/androidx/media3/demo/transformer/TransformerActivity.java
+++ b/demos/transformer/src/main/java/androidx/media3/demo/transformer/TransformerActivity.java
@@ -54,6 +54,8 @@ import androidx.media3.common.DebugViewProvider;
 import androidx.media3.common.Effect;
 import androidx.media3.common.MediaItem;
 import androidx.media3.common.audio.AudioProcessor;
+import androidx.media3.common.audio.ChannelMixingAudioProcessor;
+import androidx.media3.common.audio.ChannelMixingMatrix;
 import androidx.media3.common.audio.SonicAudioProcessor;
 import androidx.media3.common.util.BitmapLoader;
 import androidx.media3.common.util.GlUtil;
@@ -99,6 +101,7 @@ import com.google.common.util.concurrent.ListenableFuture;
 import java.io.File;
 import java.io.IOException;
 import java.lang.reflect.Constructor;
+import java.util.Arrays;
 import java.util.List;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutionException;
@@ -411,6 +414,18 @@ public final class TransformerActivity extends AppCompatActivity {
       processors.add(silenceSkippingAudioProcessor);
     }
 
+    if (selectedAudioEffects[ConfigurationActivity.CHANNEL_MIXING_INDEX]) {
+      ChannelMixingAudioProcessor mixingAudioProcessor = new ChannelMixingAudioProcessor();
+      for (int inputChannelCount = 1; inputChannelCount <= 6; inputChannelCount++) {
+        float[] mixingCoefficients = new float[inputChannelCount];
+        Arrays.fill(mixingCoefficients, 1f / inputChannelCount);
+        mixingAudioProcessor.putChannelMixingMatrix(
+            new ChannelMixingMatrix(
+                inputChannelCount, /* outputChannelCount= */ 1, mixingCoefficients));
+      }
+      processors.add(mixingAudioProcessor);
+    }
+
     return processors.build();
   }