Rear audio headphone jack not working

Hi all,

My work Mac Pro doesn't seem to be pumping any audio to the rear headphone jack. The front works, but I prefer to use the rear audio jack to keep the front looking nice and clean.

I presume this is a fault, but I noticed the speaker sounds the start-up chime even with phones plugged into the front jack (normally this fully mutes the speaker on my previous G5) so there must have been some subtle changes to the audio when they made the Mac Pro.

If it is a fault, I'll have to live with it as I can't be without my main workstation just to get this fixed, but I wondered if there was something else I should try. Audio Hijack doesn't seem to make any difference to the way the audio is routed.

TIA,

Simon.