A problem of no recording level in Audacity may occur in some versions of macOS due to Apple changing the “Privacy” settings for recording devices.
Note that although macOS refers to this setting as “Microphone”, it applies to all recording devices, including external USB / Thunderbolt interfaces.
To fix this problem:
- On your Mac, choose “Apple menu > System Preferences, click Security & Privacy”, then click Privacy.
- Click Microphone.
- Select the tickbox next to an app to allow it to access the microphone. Deselect the tickbox to turn off access for that app.
For further information, see: https://support.apple.com/en-gb/guide/mac-help/control-access-to-your-microphone-on-mac-mchla1b1e1fe/mac