I don’t own any Sonos products yet but my plan is to have the arc, sub and 2 passive ceiling speakers in a new build next year.  I haven’t seen a Sonos in wall sub. But what I can say is be very careful in a in wall sub. My home that I sold had dual in wall subs with a separate amp (not Sonos) that controlled them and the sound at times was horrible due to fibration from the wall materials. Tried to insulate behind the speakers, etc. but certain frequencies on movies and tv shows made the sound very poor.  It was very frustrating to say the least. In my opinion for me I would never do in wall again for a sub. But I get it, I went for the very clean look with in wall Sub and I assume your thinking is the same. Good luck. 

@Amazonracer thanks for the info i appreciated, i will rethink everything is mainly for music.