Hi Sean
Click Start, Run and then type in dxdiag then click OK
In the box that appears click yes
Make sure you have the latest version of Direct X which is 9.0c
Go through the tabs and do all the tests to see if any problems show up, also check to see how many sound devices you have. It is possible that a program has changed your default sound device, to a device that does not support what you are trying to do.
(Windows mixer will choose the default sound device when it is opened)
Hope this helps, and let us know the results

Bill