Never having uploaded an audio file I just checked this out.
It could either be that this is a semi-new feature added to WP or that this was overlooked by the developer. You should contact the developer and ask.
Short of the developer adding this to ACF you will need to us the WP function wp_get_attachment_metadata() to get what you want. You could also create an acf/format_value filter for file fields to add this information to the returned array.
