Two things I've noticed:
The volume pot goes bad. Wiggle it and see if you get any sound (static, etc.). Be warned it may be REALLY loud. The amp on that thing is surprisingly powerful, and if the pot's broken, you can easily end up with full volume when you don't expect it.
The amp/heatsink is bolted to the PCB and is quite big mechanically. The PCB can easily crack, usually along the line of pins for the amp chip. Often, this causes no electrical damage but breaks the solder joints. Try reflowing them.