XPS 8300: Realtek not outputting sound

I had the Realtek audio disabled for a few years while using an external USB M-Audio interface (more of a pro-grade unit). I'm using the 8300 for primarily gaming, so I wanted to go back to the Realtek, as the M-Audio was moved to a different computer.

The Realtek was re-enabled through the control panel, the M-Audio driver is disabled and the unit is no longer plugged in. The Realtek seems to be operating OK, for example:

1) The Realtek interface knows when I've plugged in headphones

2) The mixer shows audio when using Youtube or Songza, etc.

However, there is simply no sound in my headphones, plugged in the back, plugged in the front, it doesn't matter. I thought perhaps the presence of the M-Audio (disabled) driver was causing a problem, but like I said, the driver seems to be working, and sound is being generated, but just not exported through the jack. It even says "A device has been plugged into the headphone jack" when I plug in my headphones!

The only other device I've installed in the last couple years is a PCIe network adapter.

Any ideas??

Thanks!