diff --git a/libraries/transformer/src/main/java/androidx/media3/transformer/Muxer.java b/libraries/transformer/src/main/java/androidx/media3/transformer/Muxer.java
index 631ce56699..b25270942f 100644
--- a/libraries/transformer/src/main/java/androidx/media3/transformer/Muxer.java
+++ b/libraries/transformer/src/main/java/androidx/media3/transformer/Muxer.java
@@ -19,6 +19,7 @@ import android.os.ParcelFileDescriptor;
 import androidx.media3.common.C;
 import androidx.media3.common.Format;
 import androidx.media3.common.MimeTypes;
+import androidx.media3.common.util.UnstableApi;
 import com.google.common.collect.ImmutableList;
 import java.io.IOException;
 import java.nio.ByteBuffer;
@@ -26,17 +27,18 @@ import java.nio.ByteBuffer;
 /**
  * Abstracts media muxing operations.
  *
- * <p>Query whether {@linkplain Factory#getSupportedSampleMimeTypes(int)} sample MIME types} are
+ * <p>Query whether {@linkplain Factory#getSupportedSampleMimeTypes(int) sample MIME types} are
  * supported and {@linkplain #addTrack(Format) add all tracks}, then {@linkplain
  * #writeSampleData(int, ByteBuffer, boolean, long) write sample data} to mux samples. Once any
  * sample data has been written, it is not possible to add tracks. After writing all sample data,
  * {@linkplain #release(boolean) release} the instance to finish writing to the output and return
  * any resources to the system.
  */
-/* package */ interface Muxer {
+@UnstableApi
+public interface Muxer {
 
   /** Thrown when a muxing failure occurs. */
-  /* package */ final class MuxerException extends Exception {
+  final class MuxerException extends Exception {
     /**
      * Creates an instance.
      *
@@ -91,7 +93,7 @@ import java.nio.ByteBuffer;
    * Writes the specified sample.
    *
    * @param trackIndex The index of the track, previously returned by {@link #addTrack(Format)}.
-   * @param data Buffer containing the sample data to write to the container.
+   * @param data A buffer containing the sample data to write to the container.
    * @param isKeyFrame Whether the sample is a key frame.
    * @param presentationTimeUs The presentation time of the sample in microseconds.
    * @throws MuxerException If the muxer fails to write the sample.
@@ -100,11 +102,13 @@ import java.nio.ByteBuffer;
       throws MuxerException;
 
   /**
-   * Releases any resources associated with muxing.
+   * Finishes writing the output and releases any resources associated with muxing.
+   *
+   * <p>The muxer cannot be used anymore once this method has been called.
    *
    * @param forCancellation Whether the reason for releasing the resources is the transformation
    *     cancellation.
-   * @throws MuxerException If the muxer fails to stop or release resources and {@code
+   * @throws MuxerException If the muxer fails to finish writing the output and {@code
    *     forCancellation} is false.
    */
   void release(boolean forCancellation) throws MuxerException;
diff --git a/libraries/transformer/src/main/java/androidx/media3/transformer/MuxerWrapper.java b/libraries/transformer/src/main/java/androidx/media3/transformer/MuxerWrapper.java
index 32f6bf99a7..611e469b06 100644
--- a/libraries/transformer/src/main/java/androidx/media3/transformer/MuxerWrapper.java
+++ b/libraries/transformer/src/main/java/androidx/media3/transformer/MuxerWrapper.java
@@ -184,13 +184,13 @@ import java.nio.ByteBuffer;
   }
 
   /**
-   * Releases any resources associated with muxing.
+   * Finishes writing the output and releases any resources associated with muxing.
    *
    * <p>The muxer cannot be used anymore once this method has been called.
    *
    * @param forCancellation Whether the reason for releasing the resources is the transformation
    *     cancellation.
-   * @throws Muxer.MuxerException If the underlying muxer fails to stop and to release resources and
+   * @throws Muxer.MuxerException If the underlying muxer fails to finish writing the output and
    *     {@code forCancellation} is false.
    */
   public void release(boolean forCancellation) throws Muxer.MuxerException {
diff --git a/libraries/transformer/src/main/java/androidx/media3/transformer/Transformer.java b/libraries/transformer/src/main/java/androidx/media3/transformer/Transformer.java
index 9836223997..75c57ef423 100644
--- a/libraries/transformer/src/main/java/androidx/media3/transformer/Transformer.java
+++ b/libraries/transformer/src/main/java/androidx/media3/transformer/Transformer.java
@@ -373,6 +373,20 @@ public final class Transformer {
       return this;
     }
 
+    /**
+     * Sets the factory for muxers that write the media container.
+     *
+     * <p>The default value is a {@link DefaultMuxer.Factory}.
+     *
+     * @param muxerFactory A {@link Muxer.Factory}.
+     * @return This builder.
+     */
+    @CanIgnoreReturnValue
+    public Builder setMuxerFactory(Muxer.Factory muxerFactory) {
+      this.muxerFactory = muxerFactory;
+      return this;
+    }
+
     /**
      * Sets a provider for views to show diagnostic information (if available) during
      * transformation.
@@ -407,21 +421,6 @@ public final class Transformer {
       return this;
     }
 
-    /**
-     * Sets the factory for muxers that write the media container.
-     *
-     * <p>The default value is a {@link DefaultMuxer.Factory}.
-     *
-     * @param muxerFactory A {@link Muxer.Factory}.
-     * @return This builder.
-     */
-    @CanIgnoreReturnValue
-    @VisibleForTesting
-    /* package */ Builder setMuxerFactory(Muxer.Factory muxerFactory) {
-      this.muxerFactory = muxerFactory;
-      return this;
-    }
-
     /**
      * Builds a {@link Transformer} instance.
      *