I dont know if any one can help but I have just noticed I an unable to hear sounds from my desktop themes any more. I can click on them and windows media player comes up and the player bar moves along the bottom but I cannot hear anything.
It is possible that yours might look slightly different to mine (I have SB Audigy2 Soundcard), but should be similar anyway. In ControlPanel>Sounds and Audio Devices look first at Vol Tab (mine has a couple of boxes to tick as required) also under this Tab click on Advanced and check the boxes there, and also under this Tab mine has Speaker settings as well. Then look at Sounds Tab there are many boxes under this Tab to check (ie all your System sounds - you can make your own choices via the Browse button). Then check Audio Tab and Voice Tab to make sure that your default settings are correct.
Assuming you are using XP - but probably similar in other Windows types as well.