This noise is obviously electrical interference being picked up by either the internal computer sound circuitry, the speaker cabling or the speaker amplifier. I have not dealt with this sort of situation myself ever, so I will just give you suggestions that seem logical and my reasons for them.
There are two questions here. What (specifically) is generating the noise and what (specifically) is receiving it? As I see it, there are two possibilities for the generation of the noise: a bad connection and a poor design. A bad connection would be something like dirty PCI contacts sparking - I do not think that this is the case, though. Not enough voltage and it would manifest itself in other ways as well. Poor design assumes that it is your wireless card that is producing abnormal levels of electrical noise. The only way to treat this would be to shield the affected devices.
For the reception of the noise, as I said, the three possible 'vectors' are the motherboard, the cabling, and the speakers. Does the noise change when you move the speakers? when you move the cable? Do you use a sound card or built in sound? (a large parallel board would be more suceptable, but easier to shield.)
All these questions are only to help locate the source of the problem. What suggestions would I have for fixing it? Try moving the PCI card (if possible) to another slot. Try moving the speakers. Make sure that the I/O shield of the card has metal to metal contact with the case. Glue tin foil to cardboard to make a grounded shield between the wireless card and the rest of the computer. *WARNING* Make sure that there is no way ever EVER that the foil could short anything out (touch metal on a circuit board). Maybe put cardboard on both sides of it. Recess it from the edge of the cardboard. Also, make sure that it does not block important airflow. This solution should be placed parallel with the network card, be about the same size, be fairly close to it, be between it and the sound ports, and be grounded by an attached insulated wire connected to a case screw.
This is an interesting FCC booklet on television interference if you want to grasp the principles. Remember that speakers should be much less sensitive that TV's.
http://www.arrl.org/fcc/tvibook.html
And for now, I'm clean out of ideas
Good luck!