MOD to embed mp3 - help needed

Am trying to "mod" activity plus to "double" utilise the video link input field to be able to process any file with a mp3 extension and "wrap" it as follow

[audio="http://pathtofilename.mp3]

I was hoping i could tweak the code in class_bpfb_codec

/**
	 * Creates the proper shortcode tag based on the submitted data.
	 */
	function create_audio_tag ($url) {
		if (!$url) return '';
		$url = preg_match('/^https?:\/\//i', $url) ? $url : BPFB_PROTOCOL . $url;
		return "[audio='.{$url} .']";
	}

to possibly "interrogate" the file, see if is mp3 and then wrap the link with the audio= tag as shown above...

Any ideas on how i could go about this?