|I understand you said you checked all the properties but we'll double check.|
First, check in Device Manager and make sure there are no problems with any devices.
Secondly, double click on your Volume icon on your taskbar to then open up Volume Control. Make sure you're "Volume Control" and "Wave" are maxed and not muted.
Thirdly, go into Control Panel, then Sounds and Audio Devices, then under the Audio tab, make sure what's listed is your sound card. If not, select it and press ok. Same with Sound Recording.
Try playing a sound clip using the regular Windows Media Player Classic (incase settings were changed in Windows Media Player) use headphones when testing a sound clip with a working pair of headphones.
Post back with results. Thanks.
AMD64 X2 4200 @2.5Ghz
Corsair 3Gig Dual-Channel 2x512 2x1gig
MSI Radeon x800 PCI-E
Seagate 160Gig SATA
Seagate 320Gig SATA