it could also be your upper and lower filters. My mothers laptop did the same thing and I went in and deleted the upper and lower filters and rebooted the computer and it came back and it hasnt disappeared again since.
To try this you have to go in and delete them from the registry editor.
1. go to run and type in regedit
2. go to HKEY_LOCAL_MACHINE
3. go to System
4. go to Current Control Set (you might have a couple different control sets so check each one for the the next steps since some might not have step 7 and the correct step 8 folder)
6. go to Control
7. go to Class
8. then select 4D36E965-E325-11CE-BFC1-08002BE10318 (there is going to be alot that look like this one make sure it is this one)
9. once you get here look for anything that says upper filters, lower filters, upper filters.bak, lowerfilters.bak and delete them (right click on them and click delete). Once you do this close the registry editor and reboot your computer.
This is how I did it and it worked but if this does not work than it could be like sniper stated something hardware related. I would try to delete the filters before messing with anything else though since it is a very easy fix