A few things to try:
Firstly, if the computer is connected to the net, then open your device manager and right click on the offending item and "update driver" may work, may not.
This could be the result of an erraneous update being applied, if you know when it stopped working then go to system restore and restore to a previous date, this should put the drivers and system back to how they were before they stopped working.
Fail in that, then try and download the drivers from Toshiba's site, if that doesn't work then it's probably a case of a re-install of windows, and if that doesn't fix it then sadly it's probably the sound card that's shot.
Hope it helps!