Funny, I already spent 2 hours trying to figure out what's wrong, found this thread and these illustrations. I thought: wow finally, someone got exact problem like mine and even steps are exactly the same. I was ready to scroll down to see the question and answer, but then I realised the whole thread is instruction, not question or issue. I give up, I tried blackhole, soundflower, reinstalling obs, different options of OBS with different outputs, inputs, channels, bitrate, exports, different configurations of midi devices, different audio speakers but while Movavi and native recorder can capture the sound, OBS studio does not see it at all.