NO sound comeing from pc :(

[xsamx1987]

Hello all OK so my friend asked me to look at his computer it is a old dell dimension 2400 half gig of ram running windows xp sp3

OK so the problem is that his sound dosent work all the time ive checked the usual things cable- driver- conflicts- that kind of thing the weird thing is when the sound dosent work it enables the on board speaker so if I go to volume control and move the bar up and down I hear the dunk dunk sound from the on board speaker oh and the on board sound is enabled on the bios

any way home some one with a better mind then me can help