Think if you made any changes to your system, you may want to roll back.
1. Try system restore
2. Check your audio connections (speakers/headphones)
3. Uninstall the audio driver, reboot, install it, reboot (you may want to download the latest 7 compattible driver for your audio device)
4. Control Panel > Administrative > Services > Windows Audio Service (should be set to Auto) - see if it's running
5. Control Panel > System > Device Manager > Sound, Video, Game controllers > Double click on your audio device and see if there are any problems