Index: wp-includes/js/mediaelement/wp-mediaelement.css
===================================================================
--- wp-includes/js/mediaelement/wp-mediaelement.css	(revision 24938)
+++ wp-includes/js/mediaelement/wp-mediaelement.css	(working copy)
@@ -8,4 +8,8 @@
 
 .mejs-controls .mejs-time-rail .mejs-time-current {
 	background: #d54e21;
-}
\ No newline at end of file
+}
+
+.me-cannotplay {
+	width: auto !important;
+}
Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 24938)
+++ wp-includes/media.php	(working copy)
@@ -910,6 +910,7 @@ function wp_audio_shortcode( $attr ) {
 		'loop'     => $loop,
 		'autoplay' => $autoplay,
 		'preload'  => $preload,
+		'style'    => 'width: 100%',
 	);
 
 	// These ones should just be omitted altogether if they are blank
Index: wp-includes/script-loader.php
===================================================================
--- wp-includes/script-loader.php	(revision 24938)
+++ wp-includes/script-loader.php	(working copy)
@@ -569,7 +569,7 @@ function wp_default_styles( &$styles ) {
 	$styles->add( 'wp-auth-check', "/wp-includes/css/wp-auth-check$suffix.css" );
 
 	$styles->add( 'mediaelement', "/wp-includes/js/mediaelement/mediaelementplayer.min.css", array(), '2.12.1-20130724' );
-	$styles->add( 'wp-mediaelement', "/wp-includes/js/mediaelement/wp-mediaelement.css", array( 'mediaelement' ) );
+	$styles->add( 'wp-mediaelement', "/wp-includes/js/mediaelement/wp-mediaelement.css", array( 'mediaelement' ), '2013-08-01' );
 
 	foreach ( $rtl_styles as $rtl_style ) {
 		$styles->add_data( $rtl_style, 'rtl', true );