MKV file, compress or downsize audio, what app or settings?

[PetitPois]

Hello,

I recorded a MKV file with 2 audio streams and 1 video stream using OBS Studio. (Those better in the know than me might say it was a poor choice to use OBS Studio for recording this but it was the easiest app I could setup to record both headset microphone and incoming audio on speakers.)

The file is an MKV, ~4 GB big, and ~4 hours long.
Using the audio quality set correctly usually the file size is around 3-400 MB.
The audio quality is set to 1000 kbps CBR (IIRC) or something ridiculously high, otherwise normal audio.
The video is intentionally set to just a small black screen (~240x180, lowest FPS possible).

Questions
1) Is there a better free/open source app to collect microphone and speaker output? Either on Windows or Linux?
Why: OBS Studio worked, whereas Audacity and Goldwave the settings to use were not clear and I couldn't get it to record both audio streams through trial and error.

2) Are there particular settings I should use when downsizing?
Why: I've tried to do audio 'downsizing' many years earlier in Audacity from a 2 GB wav to a smaller mp3 and the audio became very robotic even at 320 bit rate, I fiddled with settings and got it acceptable but not enough so that I'd delete the 2 GB file.

[davexnet]

Open the source in mediainfo, put it in text view, and post the info here