John, I'm a sound engineer for my 'day job', and I can quite emphatically say that power line noise can cause interference in a sound system - particularly spikes which can emanate from switching a mains powered unit on or off. Such pops can cause loudspeakers to fail on high powered sound systems, which is why such amplifiers have 'soft start' circuitry which disconnect the speaker lines until the amplifier has powered up, and again on power down.

The reason for this is the Radio Frequency emissions such spikes cause. That is one reason why ferrite beads (parasitics) are often used (those plastic lumps with ferrite cores around wires), as they help suppress such interference.

Anyway, I experienced a problem similar to the one Sorcha described on my own system some years ago... if I plugged or unplugged anything on the shared (unsuppressed) power strip, the PC either made plaintive beeps, froze or reset itself.

Never underestimate the problems a 'dirty' power supply can cause!