Windows 7 - HDMI Audio Problem

Okay guys, didn't think this would turn out to be such a problem as it is:

Basically we got new computer 3-4 weeks ago - Acer Aspire X3900 running Windows 7 Professional x64 - and the GeForce 320 GT in it is HDMI ready. So we went out and bought a Full HD monitor and an HDMI cable and all was fine and dandy. HDMI controlled only the video and the standard audio jack at the back took the standalone speakers.

After not using it for a few days, I started it up today to realise that there was no sound. I checked the Sound settings and what do you know - the monitor (which doesn't actually have speakers) had replaced the default audio device via HDMI, possibly because I didn't disable the HDMI audio option in the nVidia Control Panel initially. So the only audio device I have in the Sound dialog is the BenQ G2222HD-1 monitor - nothing else (yes I'm telling it to show everything). Previously the on-board sound card + detached speakers were the default output.

DXDiag actually says there is no sound card to be found. I've now disabled audio via HDMI for the video card and still have the speaker's audio jack plugged in. It doesn't seem to detect the change in hardware when I restart, either.

I tried reverting back to the VGA plug, but all that gave me was a lower default resolution and still no working audio.

So I believe the question is: How do I go about removing the monitor as the default audio device and reverting to what I would believe to be the on-board audio jack for output?

Cheers.

The first suggestion is to use the System Restore function and restore the PC back to a time when the sound worked correctly. System Restore can be accessed from Start/Programs/Accessories/System Tools. Select a restore point when it did work. System Restore will not delete any user data or e-mails but any programs installed or uninstalled after the restore date will have to be redone.