Since you have an ADVC100, can you try this?
- Disconnect the video cable connected to the front Video Input jack
- Disconnect the audio cables connected to the front Audio Input jacks
- Take the source video and connect it to the Rear Video Input jack (the 7-pin - use the adapter "pigtail" to connect to Composite, S-Video will plug directly in)
- Take the source audio and connect it to the Rear Audio Input jack (stereo minijack, you might need an RCA-to-minijack Y cable)
- Try a capture
This will determine whether it's the audio jack itself that's finicky, or if it's something with the internal audio hardware.