My cousin's Spherex for Xbox speaker system recently developed continuous buzzing in the rear speakers. Another website had suggestions of resoldering diodes, replacing diodes, and resoldering in that area. I first resoldered the diodes, it partially fixed the problem, the buzzing became intermittent. I then replaced the 4 - 1N4007 diodes (1 Amp, 700 volts RMS max) with 4 - 1N5004 diodes (3 Amp, 280 volts RMS max). This had no effect, still intermittent buzzing. I then re-melted all the solder joints on the back of the power board, but not the small surface mount component joints, this fixed the buzzing! So it was probably 2 cold solder joints on the power board, I don't think the diodes needed replacing.
Attached are the before pics, sorry, no after pics. Notice how the diodes and resistor previously gave off enough heat to brown the circuit board. Seems like it might be a fire hazard.
Disclaimer: I take no responsibility for any injury or property damage that may occur with my described procedure. Hazardous voltages may exist. Get a qualified technician to perform any fix.