It could be a few things. First, check the volume/mute controls... Second, check your speakers. Third, if you've installed anything (service pack, etc.) recently, rollback your computer (i.e. go to Start| Programs | Accessories | System Tools | System Restore). If all that doesn't work, it may be that you need to update your sound card driver. To find out which sound card driver you need, try an online driver check such as RadarSync's (www.radarsync.com). They'll tell you which driver you need, and then you can go to the manufacturer's site and download it (or if you're not sure how to do that, you can join their paid service to get updates for all your drivers).
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