I bet you are suffering from "Sound Card Roulette". If you have multiple sound devices in your computer, the first one detected gets assigned to be the default ALSA sound device.
If the first detected sound device happens to be a webcam's microphone, well, obviously it won't work right as the default ALSA sound card. Plus, if you try to bring up alsamixer, you won't see the mixer you expect (if you are lucky enough to see anything at all).
I ran into the same problem today with a USB piano keyboard getting detected before my sound card and becoming the default ALSA audio device. A workaround is to boot Linux before you plug in things like a webcam or a midi music device.
There are some other solutions out there having to do with modprobe and udev, but candidly I don't understand those approaches well enough to evaluate them.