USB mic on Linux
USB-microphones are usually setup as /dev/dsp1 under Linux.
The stable version of Audacity will not find /dev/dsp1 unless there is also a device called /dev/dsp0. (This is a bug in PortAudio that should be fixed in a future version.)
To work around this bug, try setting the AUDIODEV environment variable (at a terminal):
$ AUDIODEV=/dev/dsp1 audacity
or try running
# ln -s /dev/dsp /dev/dsp0
as root. Please let us know whether this helps or not.