I get the funny feeling it might be more relate to EMI on the soundcard, and possibly also power differences in the line feeding the speakers coming from the wall outlet.
If you have the space on your board, try moving the sound card as far away from other components as possible. I'm thinking it might possibly be picking up interference from the motherboard's USB hubs . . .
Also, if you don't have a UPS, try plugging the speaker's power feed into a decent surge protector and see if that makes any improvement.