External iSight microphone audio not working
The FireWire iSight audio driver has been omitted from recent macOS releases.
A Mac Rumors contributor has posted a download of the driver from his backup.
Download iSightAudio.driver there, or find the same driver from your own backup.
Place the file in /Library/Audio/Plug-Ins/HAL/
In case there were any other inconsistencies from the original I used a copy from a backup of OS X 10.13. A Finder copy and authentication worked without any permissions issues.
This worked for me on Catalina 10.15.6 on a 4,1-&gt;5,1 cMP.