This frustrated me for a while, until I found a solution on ubuntuforums.org right here.
Nothing I would have ever, ever figured out myself:
Right click on the volume icon in the top panel. Choose “Open Volume Control.”
In the Volume Control, select VIA 8237 if it’s not selected.
Select Edit/Preferences in the menu, and on the dialog that pops up, checkmark “External Amplifier.”
Now Volume Control should have a third tab, “Switches.” The only setting on that tab is “External Amplifier,” a checkbox. Uncheck the checkbox. Volia! Sound!
In a way this is classic Linux insanely-weird-solutions-to-simple-problems. But we are also in a new era. Notice that the fix, arcane and improbable as it is, is entirely gui-based. No configuration files were edited, no command line utilities were run. It was checkboxes and tabs. That’s all.
It’s a different world out there with Ubuntu. They’re taking the guification seriously — so seriously that even arcane freaky fixes to stupid hardware problems are guified.
That’s pretty cool.