I've just lost the ability to hear sounds files (mp3s, streaming audio, etc.). I've plugged headphones into my speaker output and there's no sound there either. I had this problem before and someone had me delete something (I think my sound card) in control panel and one other step (that i've now forgotten) and that worked. Any ideas? Thanks.

Double click the yellow speaker in the systray and check the volume controls.

If you've disabled the yellow speaker, then go Start > Run > type in "sndvol32". Uncheck any checkboxes by "Mute" and adjust the volume level sliders up.

If it is a new soundcard You may have selected to use the digital output Find the settings under control panel

Right click My Computer, and select Properties. In the System Properties dialogue box, select the Device Manager tab, and go to (+) Sound, Video and Game Controllers. Click on the (+), to see a listing of the audio devices. If there is a yellow exclamation mark to the right of any of these devices the device is not working properly. If the above is the case it is best to start the PC in Safe Mode, remove any entries/remnants in My Computer>Device Manager under (+) Sound, Video and Game Controllers, and then reboot. Upon reboot, Windows attempts to install the driver software for the sound card. You may need to obtain the latest drivers for the sound card beforehand.