Thanks VERY much, guys. I got it up and running. A couple of notes: 1) it was easy to get audio to the external monitor (which is what the congregation would hear) by simply selecting it as the laptop's sound output in System Prefs>Sound. Of course, there may be some monitoring tweaks required this way, but this does solve the issue of streaming a service and allowing in-person people to hear things like music videos, etc. 2) But what's really cool is this can also be done wirelessly via AirPlay, so the external monitor doesn't need to be connected via hdmi, it just needs to be on the same wifi network as the streaming/OBS laptop.