Don't save as mp4, as they will irrevocably corrupt on any non-graceful shutdown of the recording process. Use remuxing after recording. - https://obsproject.com/forum/resources/how-to-convert-flvs-to-mp4-fast-without-re-encoding.78/
If you're losing audio tracks after remuxing, it's likely the fault of your media processing software not supporting the audio track and you may need to go all the way up to converting that track to a supported format. Test the videos with VLC after remuxing.
not in an automated fashion. It's fairly easy to get it to work by following the ffmpeg method of the remux guide and specifying an audio codec and its appropriate parameters instead of using the acodec copy parameter.
I linked it in my earlier post.