Xbox 360 sound distortion through PC

i'm currently using a trust 5.1 Surround Sound Card SC-5250 / 514DX 5.1
Sound Expert Optical Dolby (r) Digital 5.1 PCI sound card with digital
and optical audio in and outputs and i have my xbox
360 connected to the sound card via the optical input. the problem i've got is that when i am on the xbox 360 start up screen or load a game up game up i get alot of sound distortion through my Creative Gigaworks G550W 5.1 speaker, but its ok when i'm watching a film through windows media center. my operating system is windows vista 64, i've tried updating the drivers and that but nothing works.has anyone got any idea how to fix this problem?