I had some troubles with my external sound card - Creative SB MP3+.
I was able to get the external to run on debian using other applications for recording and listening to sound, but was unable to use either ffmpeg or xvidcap to capture sound.
Has anyone had any success in using an external usb sound card with xvidcap? In the end, I used an internal, which worked perfectly first time out. However, I am curious about the external issues I had and wondered if there was anyone out there with any solutions.