Considering how easy it is to mux soundtracks into an mkv using mkvmerge, assuming they are synchronised, would it be better to include just a few core soundtracks in the mkv and provide the remainder for download individually, so the "customer" can decide which they want?
This would also permit improved or additional soundtracks to be made available when they are ready and not hold up the basic mkv.
From my perspective, there are probably soundtracks envisioned for the mkv that I wouldn't be interested in listening to and they will take up more download space than necessary in the mkv.
Anyway, just a thought.