Audacity would grab the digital data for recording in this case and since it uses a lossless format for recording you would not lose any information. I don't know about OBS but it would be surprising if they do things differently.
There will be a very slight loss of quality with 320 kbps mp3s. I doubt any human would hear a difference, though. This may ...
The solution at question " Lower framerate without re-encode " looks promising:
ffmpeg -discard nokey -i input.mp4 -c copy -vsync 0 I-only.mp4
By manpage there should be 5 levels of frame discarding: none, noref, bidir, nokey, all.
I tested this with older ffmpeg 4.2.4 and h265 and it had no effect. Probably not implemented yet.