Tips for using a USB Mic on Linux: Audacity
This is primarily focussed on the popular Samson C01U range, and is partly for +Peter Cannon who I know has had some issues with this mic, but this might be useful for others as well.
Firstly, there's a need to unravel some of the mess in the various Linux audio systems and the associated apps.
The hardware-level drivers for audio on Linux is ALSA, and every app that uses audio will use these either directly or indirectly. Unfortunately the support for ALSA isn't always as complete as it should be.
If we consider the popular cross-platform app Audacity. This does work directly with ALSA, but the (badly named) "input volume" control doesn't seem to work properly. Ideally this should control the input gain (correct term) of the audio input via the ALSA driver. It's not the only app that has trouble with this it has to be said. The popular Pulseaudio sound server which is used as an audio abstraction layer on many modern desktop Linux distros, also doesn't.
The trick here is to use an app which controls the gain by talking directly to the ALSA driver. My favourite here is QasMixer (seen below). With this you can select the specific mixer device (in my case I have selected "hw" which indicates to use the ALSA driver directly) and the specific card.
You can also use the command-line tool alsamixer, but that's a little unfriendly for many people. The nice thing about QasMixer is you can it in a window alongside audacity. One small problem here is that if you click on the QasMixer window to drag the control, it moves the focus away from Audacity which then hides the meters. However, by by hovering over the QasMixer control with the mouse cursor and using the mouse scroll wheel, you can can change the gain without actually clicking on the window to highlight it.
In Audacity, set the input device to "Samson C01U: USB Audio (hw:x,0): Front Mic:1" and set the number of Input Channels to 1 (Mono).
Note that the C01U presents as a stereo device, but only the first (left) channel should be used.