Try clicking on the sound icon on the top right and selecting 'Sound
Preferences' then male sure under the 'Hardware' tab the selection is
for 'Analog Stereo Duplex' and under the 'Input' tab select the input
you are using ( particularly if using the USB device ). Might take a
bit of trial and error but once you've got the correct setting you'll
not have to change it. Had to fiddle a bit like that myself to get
Audacity to accept input !!