Loss of wave file volume setting

My client has a Windows XP desktop with a
Realtek on board audio chip. Recently, he
has been getting no sound when loading the
OS or when closing it. I checked the audio
settings and the volume for wave files. It
was at 0. I reset the slider to maximum and
tested sounds. All were fine. I rebooted
the PC. Windows startup audio played. When
I shut down the PC the Windows closing audio
played. The next time I powered up, the
Windows startup audio did not play. I then
checked the settings and the wave volume
slider was again at 0. Tried resetting
several times. The same sequence occured.
All other settings remained where they were
set. Have never seen this before. Any
ideas?