How do I find out where in /dev my microphone audio input is going?
Look into /dev/snd/ in order to find the individual devices. /dev/snd/ is a directory, not a file.
Which version of xvidcap do you use? Mine, 1.1.7, has no option to set the sound input other than the system default. Ideally, xvidcap should use PulseAudio for audio input, failing that it should use Alsa. The /dev/dsp and other files in /dev are remnants from OpenSoundSystem (OSS), which is no longer supported (since Ubuntu 10.10).