The card could be going bad. Try to see if you can test it in another system. If it does the same thing, the card is bad.
Also, try testing your speakers. Either use another system or, if they just have a single green plug, connect it to an MP3 player or something like that and see what happens. If the problem still occurs, it's your speakers that are bad.
If neither, it could be the motherboard. I suspect the card is PCI (I've got a similar one) so if you've' got another PCI card, put it into that slot and see if it works.
Intel Q6600, MSI P6N 650i, 2x1gb OCZ DDR2 800 RAM, Sapphire Radeon 4870 512mb
1x112gb Samsung, 1x320gb WD, 1x160gb WD
1xSamsung DVD/CD RW drive, PC Power & Cooling 750W PSU, X-Fi Fatal1ty Champion, Logitech Z-560 THX 4.1, NZXT Zero, G5, VW222U 22" monitor
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