Using a triac to switch audio signal for laser maze
Good question, but at the same time this circuit will probably not work. Things are a bit more complex than you foresaw. Three reasons:


The photo diode will not be able to drive a TRIAC directly. As you mention yourself, you need something amplifier/driver stage.
A TRIAC will drop at least 1.2V, leaving very little signal at the output.
The audio signal will not be able to source/sink the hold current for the TRIAC.


What would be simplest is to use a relay, as that is easy to drive and your audio signal won't be influenced much. You also need a detector, relay driver and probably a one-shot timer to leave the music playing for a brief moment after the detector is hit. Designing such a circuit is -in my opinion- beyond the scope of this question.