Audio only works sometimes - reboot required

I've got a problem with the audio on my pc. Here is the
problem:

From time to time (becoming more and more often now) when I
start my PC, the audio isn't working.

I've checked the volume and drivers, and they're ok. If I
try to play a sound, the player just hangs.

Sometimes, when I reboot, it works straight away, sometimes
it can take up to 20 or more re-boots before it comes on.
It's driving me up the wall!

I've checked in BIOS and I don't have any on-board audio
interfering.

Any suggestions??

Thanks in advance.