At first I thought it was the sound chip on my motherboard, so I put in a Creative Audigy 2 ZS. The audio was certainly better quality, but just as quiet.
I rebooted today and the audio was very loud at the half-way mark like it should have been earlier. I decided to change the bass level in the sound properties and as soon as I moved it a pixel to the left, the sound got quiet like it was before.
So it's apparent that it's a software issue, not a hardware issue.
I'm wondering what could be causing this?