Weird sound problem

I have a homebuilt pc (E6750, 8800GTS, Gigabyte P35-DS3P mobo, 4Gb RAM, Soundblaster X-Fi gamer, Vista Ultimate etc).

Any sound which is constant (eg applause) fades in and out about once a second producing a phasing effect. I have tried the on-board sound and a soundblaster X-Fi and it happens with both. I have all the latest drivers installed. I have Logitech 5.1 speakers.

Any ideas please?
Thanks.

Could be the connection with the center channel and the mobo/speacker. Check these (I have the X-540s, and the connection goes all the time, giving the impression that the speack fades away)