External Input via 3.5 mm audio port on 2022 MacBook Air

I am combining a backup track and piano playing in a Behringer mixer and feeding that to my MacBook via the audio port. I have checked the line and the signal is there but my Mac is not picking it up. I have chosen 'external mic' in System under Sound inputs but that doesn't work. Is there something else I have to set up to get my Mac to recognize this input and combine it with video using Photo Booth?