www.makemkv.com

So I ripped a UHD Blu-ray and I played in Plex on my windows 10 pc. While playing, I noticed there wasn't a stereo audio track available. There was only 7.1 and 5.1 audio. I have a stereo set up so I am missing important parts of the audio. Is there a way to add a stereo audio track without compressing the video file?

If your playback device doesn't downmix the audio to stereo for you, or if your playback hardware / software doesn't support downmixing, you could use software such as Handbrake to create a downmixed stereo track. Once you create a stereo track, you could mux a new MKV adding the downmixed stereo track and keeping the original uncompressed video by using mkvmerge.

You can do this with MakeMKV - it has audio down-mix feature. Enable expert mode, and select AAC-stereo profile . For each audio track you will have an option to save the original AND/OR stereo copy. And you can do this for already existing MKV file, just open the MKV file in MakeMKV.