Actually, I recorded my first and so far only screencast with a usb headset. But when I try recording with USB external soundcard or a built-in soundcard using PulseAudio, I can never get sound from "monitor" recorded. In fact, it is never shown in PulseAudio Volume Control. I can hear it through the speakers, but it is not shown there. And, of course, when I turn recording on - nothing is recorded, just silence.
At the same time, when I record with my USB headset, I can see signal in the monitor in PulseAudio.
Obviously, other cards just don't get their monitor sound across to the PulseAudio.