Sound at startup but not for long!! (on XP)

Hello,

Windows XP SP3

I've got a really strange problem here. As far as I can tell, each time my sons try and play a game on the PC - or when trying to play an audio CD - the sound stops coming from the speakers. It is still okay through the earphones though.
So, start up the windows jingle plays fine through the speakers.
If I go to YouTube and play a video the sound is still fine.
If I go to BBC website and play a video the sound is still fine.
but
if i play an audio CD there is no sound from the speakers (but fine through headphones) and I have to reboot to get the sound back again.
or
if a game is played that requires a DVD there is no sound from the speakers (but fine through headphones)

I don't know if this will help but the DVD/CDROM drive is
HL-DT-ST DVD-RAM GSA-H20N
Driver Provider - Microsoft
Driver Date - 01/07/2001
Driver Version - 5.1.2535.0