function playTrack(sound_name, volume) {
	current_track_id = sound_name;
	return play(current_track_id, true, null, volume);
}
play the specified audio track<br>
this function automatically set the loop property to true<br>
and keep track of the current sound being played.
@param {string} sound_name - audio track name - case sensitive
@param {number} [volume=default] - Float specifying volume (0.0 - 1.0 values accepted).
@returns {number} the sound instance ID.
@example
me.audio.playTrack("awesome_music");