My cell phone causes some severe interference with my speakers. The buzz from the speakers gets so bad when I receive a call that I actually have to leave the room.
I twist-tied the speaker wires to make them as short as possible which helped a little.

What else can I do? Would spliting and twisting the wires make a difference? I have M-Audio LX2 speakers. The speaker wires are those 2 wire ones with bare wiring on both ends. Isn't twisting supposed to reduce interference?

You could try to get some ferrite beads. The problem is most likely in the connection between the computer and the speakers, not the speaker wire itself. (Unless you are using a digital line.) Put the bead on the end of the cables closest to the amp. They're more effective if you pass the cable through once the bead twice, wrapping it around the bead. If that's not effective, you'll have to get a shielded cable.

Radio shack should have ferrite beads. On the internet, All Electronics is a good store to get that kind of small stuff.

If you know what frequency the rfi is, you can get beads that have better performance at that frequency. What kind of cellphone do you have?

Maybe, you leave his cell phone very near of your music system (PC + speakers); as a result of this, he interferes with your speakers.

Try leaving its cell phone far from his music system, but in the same room.. probably the problem is corrected of this way.