We ran into this issue ourselves and found a solution here:
Basically, in order to solve this you need a new copy of AU Lab on your Mac. This is what we did:
- Zip AU Lab
- Email the zip file to yourself.
- Download the zip file from your email.
- Unzip and launch the new copy of AU Lab.
AU Lab should ask you for permission to access your microphones. After granting permission the copy in your /Applications folder should work as well.