In Wheezy this doesn't seem to be possible, and it took me a while before I came across a solution on Stebalien:
- Install pavucontrol and the gnome-sound-recorder.
- Open the gnome-sound-recorder and start recording
- Open the pulseaudio volume control and switch to the recording tab
- Click on the down arrow of the "gnome-sound-recorder" Record Stream, Select "Move Stream" and move the stream to the "Monitor" stream for your sound card.