I think it depends a little on the sound card you use. I have managed to record both sides of a Skype call using just the ALSA tools, although there was a change I needed to make to the mixer settings before it would work. I think this process only works with programs that use ALSA (as opposed to OSS), and I don't know anything about this mumble thing you mentioned, so it migght not work.
Anyhow, for what it's worth, I wrote it up here
The comment in there about Audacity not supporting ALSA is out of date - I believe the audacity which ships with Ubuntu 7.10 does not support recording from the ALSA device, so you should be able to use it.
The most important information is that you need to select both the MIX and CAPTURE channels in the mixer.