When it starts do you get the same scratchy sound if using windows media player or any other music playing software ?
Also when it happens are you listening to it though the computer speakers or an external source ?
If it happens while listening via the computer speakers try plugging in some headphones to see if the sound improves, visa versa if it happens when listening through an external source disconnect and see if it's the same via the computer speakers.
Are you using an external sound card or the on board sound card ?
Did it start happening after installing any new software or upgrades.
If it only happens when your computer has been on for a time it may be a component breaking down with temperature.
If this sounds a possibility try an external sound card for a time to see if that improves it.
You can pick up some cheap little usb sound cards from amazon just to check if it helps like this:
http://www.amazon.co.uk/Dynamode-Cha...998881&sr=1-14
This is the amazon UK I'm sure you can find similar for any other area.
If that improves the problem then you can always upgrade to something beta later, don't be put off by the size and price I have one of these although I mainly have it for monitoring. Although it says 7 channels take no notice of that just use it as 2 channel sterio.
It will save you laying out a lot of money just to pinpoint your problem.
One other thing, could your computer be overheating ?