To find your soundcard try this ...
Start > Run > dxdiag > Sound - your device make and model should be displayed
Connect a pair of headphones to your souncrard instead of your speakers just to confirm that it isn't a speaker problem.
Try uninstalling all the 3rd party software that uses your sound card such as Quicktime, Winamp and Flash etc.
Uninstall your soundcard drivers then reboot.
When the system has rebooted install the latest drivers for the sound card and see how the system reacts, if everything is OK reinstall your applications one by one with a reboot and test between each install.