function unload(sound_name) {
	if (!(sound_name in audioTracks)) {
		return false;
	}

	// destroy the Howl object
	audioTracks[sound_name].unload();
	delete audioTracks[sound_name];
	return true;
}
unload specified audio track to free memory
@param {string} sound_name - audio track name - case sensitive
@returns {boolean} true if unloaded
@example
me.audio.unload("awesome_music");