My NVR 7616Ni-i2, also has a BNC input & output for audio.
Haven't found any mention of this in the manuals, and I don't see options to enable it in the web interface.
Surprisingly, nobody ever mentions that and we've never explored it.
I would have thought it'd be covered in the manual, but as you say I can't recall ever coming across anything in the user interface that relates to it.