I have a similar problem.
I installed Fedora7 on my new Dell Laptop and my USB headset (Plantronics DSP300) seems to be working fine.
The problem is, as soon as I try to change volume level all USB devices stop working. For example, right after booting I go to System -> Administration -> Sound card detection, play a test sound (works), try to lower volume level and bang! sound stops and the mixer_applet2 process goes in an "uninterruptible" state (and, when I reboot, the shutdown process hangs at "Saving mixer settings").
FWIW, I always let package manager do its updates when it finds some.
Oh, and... did I mention I'm a Linux newbie?
TIA for your assistance.