This sounds like the classic mobile phone situation, when the cell checks to see who's in the area and how far they are. The "is-anybody-out-there" check happens at regular intervals, and the tower and mobile use full strength to ensure they find each other and can then work out how far apart they are, so that future calls (for the next 10 minutes) can use lower power. If your computer speakers (or your computer) aren't well shielded or if the cable to them is a bit dodgy, they'll pick up the transmission. Actually it's the amplifier bit that picks it up, so you may also notice the same behaviour from your hi-fi or any other audio equipment. That's why it can happen if the PC's off but the powered speakers are still switched on.

The simple solution is to put your mobile somewhere else. The alternative is to get better computer speakers which are properly shielded, and not the crappy $2 Walmart rubbish which most PCs come with.

Graham.