Mixer outputs in mono only to Audacity or Garage Band

Apple folks, I have two podcasting students who have Macs - different laptops but same version of the OS (latest I think.)

One plugs in his board (a small 2 channel mixer) and he gets a stereo track in Audacity, the audio recording software. The OS automatically picks a USB codec that is able to be recorded in stereo. The other student's board is more sophisticated (8 channel) but when he plugs in (using s similar USB connection) he can only get mono.

For some reason, his laptop is choosing this mono-only codec. We've tried to change the settings in the AUDIO setting tab but there is no other USB codec beside this "USB Headphone Set."

The odd thing is, when I plug in an audio interface (basically a small mixer by Pyle) it (the laptop) chooses a USB codec that can be recorded in stereo. I can't remember the name of it but it's different from the other codec.

Any ideas on how to force the laptop to use the correct codec and not autochoose one that is mono?

I should also say that the software doesn't matter - GarageBand gets exactly the same results - mono only. It has to be a missing codec or for some reason the Mac is seeing this as a mono input and choosing the codec accordingly.

I'm sorry for my ignorance - I'm a PC guy. I can fix this in a PC but in a Mac I'm stumped.

Any suggestions are appreciated.