The Microsoft introduce a new feature in the Windows7 that is if your computer are in sleep mode there is no sound in the computer which is a nice feature.
That information is not helpfull to the problem.
I have to say its a driver/device issue. I had that before. One work around I found was not to put it in sleep mode but put it into Hibernation mode.
It can be either the driver software is corrupted or the actual sound device is dieing.