Rewrapping mkv to mp4 but insert different audio track

[dmjerbear]

I've got ~700 DVDs and Blurays ripped to MKV. Good video quality, 5.1 AC3 passthrough using MakeMKV and Handbrake.

I'm adding a NAS so that all of those movies can be accessed by anyone in the house. To avoid transcoding as much as possible, I'd like to do the following:

1. demux the mkv
2. rip just the stereo track from the original bluray. Change to AAC format if needed.
3. remux the original video with the new audio track into mp4 format.

What tool(s) would allow me to do this? My biggest stumbling block so far is ripping just the audio from a bluray. Thanks in advance!!

[jagabo]

mkv2mp4