Play Audio in WordPress Excerpt

functions.php

<?php
/*Add this code to 
functions.php

From: http://premium.wpmudev.org/forums/topic/audio-preview-in-product-excerpt#post-280140


*/
function add_audio_to_excerpt( $excerpt ){

  // get post
	global $post;

	// have post ID?
	if ( !empty( $post->ID ) ) {

		// get audio
		$meta_value = get_post_meta( $post->ID, 'audio_source', true );

		// have audio?
		if ( !empty( $meta_value ) ) {

			// do shortcode
			$shortcode = do_shortcode( '[audio src="' . $meta_value . '"]' );

			// have shortcode?
			if ( !empty( $shortcode ) ) {

				// append shortcode to content
				$excerpt = $excerpt . $shortcode;
			}
		}
	}

	// return excerpt
	return $excerpt;
}

// filter excerpt with above function
add_filter( "get_the_excerpt", add_audio_to_excerpt );

Hello friend,
I was using this great code on my theme, until I faced a huge "Error Log" on my cPanel host, saying "PHP Warning: Use of undefined constant add_audio_to_excerpt- assumed 'add_audio_to_excerpt' (this will throw an Error in a future version of PHP) in /public_html/wp-content/themes/Impreza-child/functions.php on line 74

Do you have any idea how I can fix this error?

Thanks