Have you recently changed your sound setup or installed any new drivers or audio applications? In the past I've had similar problems following driver updates where volume levels would reset themselves every time I restarted the machine - Reinstalling the driver generally fixed the problem though I never knew exactly what it was that went wrong.

If possible, please give information on your audio hardware as well as the driver version. It may be a known problem in which case we can look at a solution, otherwise we may be able to point you in the direction of new drivers which will hopefully solve this little niggle.