SN27-P2 Audio Levels varying

I've recently purchase the barebones unit secondhand to use as a Media Centre, and installed a new Processor, 4 Ghz Memory and Video card, but I've noticed a problem when playing back music using Vista Premium Media Centre.

The Audio levels are constantly changing, after its been running for an hour or so. I'm using the onboard sound system as supplied. I've considered upgrading to a PCI sound card such as possibly M-Audio 192, but I'm happy with the general quality of the onboard system, if the level problem can be resolved.

Any Help or Suggestion

Cheers
Bob