If you have sufficient disk space, why not just capture both audio and video, then, take the clip to the timeline and delete the video track. Voila! Audio only.
I do this quite regularly when I want to burn old LP's to CD ROM. Record the sound to your camcorder (with lense cap on or off as you prefer) and then shoot both across the firewire via Vidcap30. Nothing to it.
GG:
What I do is capture video (even if it's blank) and audio, delete the video track, then render audio as a wav, save the wav, delete the original avi, and the "wasted" disk space is recovered.
Rendering is a definite extra step, but, I've found that I have less hassles in recording audio if I do it this way.
I then use VV or WaveLab to further edit the resultant wav before burning to disk or outputing to cassette or whatever.
In this way, everything stays digital (at least until I output to analog tape).
To me, I find spending the extra time to render the avi to wav is preferable to acquiring the audio via my computer's analog inputs. If my source material is good, I never have to worry about overloading or levels, etc.
You could also open the captured 'video'/audio file in the trimmer and then elect to place audio only on the timeline (although I can't see where that is so much faster than just dragging everything to the timeline, then deleting the video track.
Anyhow, I recently purchased the entire vinyl LP collection of a local university (.10 per disk . . . and this was a "faculty library", non-circulating collection, so the LP's are in great condition), and have been doing a lot of "audio-only" capturing. The above works best for me.
I use my DV Camcorder as a high quality audio recording device hooked to a mixing board. I just want to capture the audio clean and edit... and export an mp3.