Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 40723)
+++ wp-includes/media.php	(working copy)
@@ -2161,7 +2161,7 @@
 	 * @param array $extensions An array of support audio formats. Defaults are
 	 *                          'mp3', 'ogg', 'm4a', 'wav'.
 	 */
-	return apply_filters( 'wp_audio_extensions', array( 'mp3', 'ogg', 'm4a', 'wav' ) );
+	return apply_filters( 'wp_audio_extensions', array( 'mp3', 'mp4', 'ogg', 'm4a', 'wav' ) );
 }
 
 /**