Ableton Live was getting any sound from the mixer and I thought I was doing something wrong. I completely forgot that this also happened with Reason. Apps need to be granted permission to use the microphone (which includes all audio inputs) but Live hadn't prompted.
As previously, SIP (System Integrity Protection) is disabled preventing apps from requesting access. This is likely due to using OpenCore Legacy Patcher.
Rather than re-enable it I used the workaround to give Live permissions by adding it to the TCC database (Transparency, Consent, and Control) via the following commands in terminal:
> sqlite3 ~/Library/Application\ Support/com.apple.TCC/TCC.db
> insert into access values ('kTCCServiceMicrophone','com.ableton.live', 0, 2, 2, 1, null, null, null, 'UNUSED', null, null, 1669648527);