How can I stop audio equipment humming?

likely its RF interference, either from the electricity or stray electromagnetic radiation

you can get RF filters for some of the cabling, and just move them until they stop.

Most 'el-cheapo' entertainment centers use the lowest

quality parts and engineering possible to keep their costs

of manufacture down. What you have is a power supply

(internal) that has too low a value of capacitance across

the positive (with reference to ground/common) causing

a faint hum in your speakers. No RFI filter will fix the

problem. What you need to do is unplug the unit, remove

the case and locate the Filter Capacitor. It will be the biggest

Capacitor on the printed circuit board (found in the power

supply section). Trace the secondary wires from the power

transformer to the PCB. The capacitor closest (and biggest)

to the rectifier will be the trouble. It is a good cap; but we

need to use a larger value. You can solder one (of the same

value) in parallel with the old one on the solder side of the

PCB. You can keep the leads of the new cap long as this

will help in soldering it in place. Note the polarity! If you reverse

the polarity; the capacitor will vent, leak and/or explode!

Doing this properly will quiet down your speakers. You can

find a capacitor at Radio Shaft (or similar store) that's the

'fast food of electronics'. Feel free to e-me.

Try a different power socket and see if the hum goes away. There are instances of local interference causing hum noises, for instance a friend of mine gets a variable strength humming noise in his PC speakers from October to April which he suspects is industrial heating but has been able to track it down.