Dell :: Possible Unique Sound Problem With Studio XPS 16 (1640)

Long story short:

My sound does not play through my on-board speakers, but does play through external speakers or headphones. It is not a hardware issue, as I can replicate a way to make sound play through the on-board speakers temporarily, but I cannot figure out a permanent solution without doing a fresh install, and I'd rather not resort to that.

Long story:

About a month ago, my original hard drive failed. I was able to request a replacement hard drive under my warranty, which came with a self-installing image of Vista (and other Dell software). I was using Windows 7 beta on my old hard drive and decided to go with the release candidate for my new one. I bring this up because it seems I made a mistake in upgrading from Vista to Windows 7, instead of just using a clean install. On my old hard drive, I did a clean install of the beta and had to do some tinkering to make it all work, so I was hoping I wouldn't have any issues with drivers by just upgrading then removing the excess bloatware.

Well, everything seemed to go according to plan. I installed a bunch of programs, copied a lot of stuff from an old PC, and didn't suspect anything until, one day, I didn't have my external speaker jack plugged in. I wasn't hearing any sound, but knew that my on-board speakers should at least be working. I first thought I may have disconnected something when installing the hard drive somehow, but the hardware checked out just fine. I uninstalled, reinstalled, upgraded and did a number on my drivers, but nothing seemed to work.

I know that it is not a hardware issue, because I can replicate a way to make sound play through the on-board speakers temporarily. If I open up the sound options from the control panel and "test" both of the enabled devices while listening to music, my on-board speakers will temporarily play whatever music I'm listening to, and my external speakers will play the test sounds. It seems to be a weird issue where all sound is being directed externally, unless a test in progress, then the sounds that are already playing will be projected by the on-board speakers. To make it simple: the on-board speakers work if I run a test while listening to music, but only for the duration of the test (3 seconds), then the problem returns.

As of now, everything is exactly how I want it to be on my machine, and I don't want to have to do a fresh install just to fix this minor-yet-annoying issue.

Relevant information:

OS: Windows 7 Release Candidate 1

Drivers on Device Manager:
ATI High Definition Audio Device
IDT High Definition Audio CODEC
(tried all of the latest IDT drivers, also tried Windows automatic drivers)

Sound Devices from Control Panel:
"Speakers / Headphones" enabled as Default Device
"Independent Headphones" enabled as Default Communication Device
(I have tried switching their roles, enabling, disabling and testing them all)