I recently upgraded from Ubuntu 11.10 to 12.04. My Samson C01U USB microphone, which has worked for many years on Ubuntu, now is having problems.
The symptom is as follows. After a reboot, the microphone works as expected. Recording with Audacity is fine. Then, after some indeterminate amount of time, the microphone disappears from the system. That is, it is no longer listed in pavucontrol or alsamixer. Unplugging and re-plugging the microphone does not resolve the problem. Occasionally, the microphone will appear in pavucontrol for an instant on re-plug, but disappear immediately.
I have disabled PulseAudio and observed the same behavior. (That is, the microphone works fine, then after some time is no longer a valid "card" in alsamixer.)
The only relevant-looking line in the syslog is as follows. The timing correlates to the disappearance of the microphone, and the USB device number is that of the microphone.
kernel: [63194.088077] usb 2-4: reset full-speed USB device number 12 using ohci_hcd