I plugged it in hopefully when i got home. The right speaker still sounded good, but the left speaker emitted nothing but a loud 60-Hz hum. The left speaker is just passive; all the electronics and controls are in the right speaker. With nothing to lose, i reached for a long Phillips screwdriver and started taking the right speaker apart.
So how can a fall to the ground break a circuit that's made of components soldered to a printed circuit board? The board itself wasn't cracked, and the capacitors and resistors stuck to it wouldn't have been affected by shaking. But there are two chips screwed into a huge heatsink.
Those chips are TDA2030 audio amplifiers. I checked out the one connected to the output for the left speaker. Stupidly, i plugged in the power and tried wiggling the heatsink to see what would happen. The left speaker made all sorts of scratchy noises. So i looked at the bottom of the board and sure enough, when the speaker hit the ground the weight of the heatsink had caused two of the legs of the chip to tear their traces off of the board. Yay! Out came the soldering iron and the solder. Nothing like the smell of melting solder to bring me back to my Waterloo engineering days.
The little W-shaped cluster of five silver blobs to the left of the black screw is the group of legs to the amplifier chip. The bottom two legs had come off. I patched in a couple of lead wires to reconnect the chip's legs to the busted traces, and now i have lovely lovely stereo sound again.