custom field (video/audio url) and embed functionality

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As you are using URLs from oEmbed provides (YouTube, Vimeo, Soundcloud, etc), I would use wp_oembed_get() function. This function uses oEmbed and tries to get the embed HTML of the provided URL.

$embed = wp_oembed_get( $some_url );
if( $embed ) {
   echo $embed;
} else {
   // The embed HTML couldn't be fetched
}