Hi, this is my second day with OBS and I know how you feel about no audio. I tried everything too but I found most instructions were ambiguous and skipped steps for a first time user. Just because you may have used a similar app, before, does not mean that the new one will follow the same work flow. They recommend downloading and installing an app, iShowU Audio Capture. No matter what I did, it would not function. Delete it.
I have a little experience with audio interfaces and Macs and know several idiosyncrasies regarding audio. I applied some of this knowledge and downloaded and installed Soundflower. At this point do a restart. I am running OS 10.14 6 Mojave. GPU 8 Gb and RAM 32 GB, new 27" iMac.
So, I load my video to capture on screen, open OBS and setup settings.
In system prefs,/sound, check sound flower 2Ch as output and set level to about 50%.
Your OBS video settings should be OK as you are obviously getting a picture.
Record a little, save and playback. Make sure in system prefs,/sound, you recheck output to internal speakers for playback. This all worked for for me. I see you are using an HDMI hub. Not sure if this patch works in that setup.
