Electronic – Using optoisolator and triac with mcu to switch light on/off–need low power alternative

Good question, but at the same time this circuit will probably not work. Things are a bit more complex than you foresaw. Three reasons:

1. The photo diode will not be able to drive a TRIAC directly. As you mention yourself, you need something amplifier/driver stage.
2. A TRIAC will drop at least 1.2V, leaving very little signal at the output.
3. 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.

Are you sure whatever is driving the MOC3010 can sink 15 mA? The MOC3010 is only guaranteed to switch at 15 mA forward current through the LED. The optriac may switch at lower forward currents, but the apparently Motorola bins those parts by forward current sensitivity, and the 3010 is the lowest sensitivity.

_{IFT = LED Trigger Current, Current Required to Latch Output}

With a footnote:

All devices are guaranteed to trigger at an I_F value less than or equal to max I_FT. Therefore, recommended operating IFlies between max 3.IFT(15 mA for MOC3010, 10 mA for MOC3011, 5 mA for MOC3012) and absolute max I_F(60 mA).

What is driving the LED? Can you measure the voltage on the IO line going to the optocoupler?

It's also worth noting that even with the microcontroller completely out of the picture, you'll only get 10 mA forward current with a 330Ω resistor (\$5V - 1.5V_{F} = 3.5V\$ across the resistor. \$\frac{3.5V}{330} = ~10.6 mA\$).