Enter "default" to capture from the default sound card.
If your webcam has a microphone that is separate from the main soundcard, enter
into a terminal window to get a list of audio capture devices. The list will look similar to this:
Each card has a number and a name. In the example above, the card name is "Intel". Find out the name of the webcam microphone card, then in VLC enter
**** List of CAPTURE Hardware Devices ****
card 0: Intel [HDA Intel], device 0: STAC92xx Analog [STAC92xx Analog]
Subdevice #0: subdevice #0
Subdevice #1: subdevice #1