Hi, yes, I already asked few weeks ago about the same problem. The OBS can, in my case, see the sound card as well, but it is not producing any sound/noise etc.
I tried various settings, also play with MIDI settings etc., but nothing helped. Even any of the programmes like Loopback, iShowU etc.
Today, after a month, I tried to install an update of OBS, but nothing changed.
How do you mean that? At least in my case, when the external card is plugged in, the Mac can see it. And also plays the sound, e.g. through GarageBand app, but OBS is not picking it up. Neither, when you choose the sound card or something like "aggregate device" as the main sound input.