this happens to be me too with this notebook on Windows 7... I think it's something to do with the Windows Audio Service personally because all i haveto do to fix there being no sound is go to
(no headphones plugged in)
1) Start Menu
2) click on Search program and files and type in 'services' then click on Services when it comes up
3) in Services look for Windows Audio service
4) Stop Windows Audio Service
5) Start Windows Audio Service
then the sound will work again.. you will probably have to restart the app using the sound for the sound to work in that app again though as it has to renegotiate the communication with the sound card.