First go into Device manager and find the items that may have a yellow exclamation mark next to them and uninstall them, then reboot, when windows starts it should reinstall any lost drivers, you may need to have the driver disk at hand if you dont have it download the relevant drivers from the manufacturers website.
Verify that the sndvol32.exe file is located in the system32 folder under C:\\windows\. If not install it there from your CD or download it online. Then go to the control panel and click on "Sounds & Auido Devices" put a check beside "Place volume Icon on task bar."
Go to Start --> Run, and type in services.msc.
A window will pop up with a list of services. Scroll down to Windows Audio. Right-click
and click on Properties.
Make sure that "Startup type" is set to automatic. Also, make sure, under "Service status," start is selected.