Audio post format with Advanced Custom Fields

Advanced Custom Field plugin uses get_field() to retrieve values.

Your code should be something like:

$audio = get_field('audio_post_format_url', $post->ID); 
var_dump( $audio ); # To make sure you're retrieving the right value
echo wp_oembed_get( $audio ); 

Note that get_field() does not need a second parameter, it will take the current post ID if omitted.

If var_dump( $audio ) displays false, then you should check your ACF key.