I think you have a couple problems with this setup.  One is that it looks like these Multiview switches are generally lower video quality.  I did a quick look and the best I saw was 4k @30Hz, which is ok, but not what I would want to be limited to if you’re getting a projector.  The second is that these things don’t see support audio beyond just stereo. 

One device that could help you is an HD Fury Arcana, which can convert audio from normal HDMI to HDMI-ARC, while passing normal HDMI video. This would be ideal for your projector alone, since your projector likely doesn’t have HDMI-ARC.  However, not sure it helps when you add the multiview switch to the setup.  If you use the Arcana between the swtich and the projector, audio has already been downgraded to PCM, as I understand it.  It’s too late.  If you have Arcan between your sources and the switch, you would need 4 Arcana, one for each source...or the ones you want to hear audio from at least.  I’m not 100% sure that would even work though as the lower video quality may mean you can’t get Atmos audio with the same movie/program.  Not sure on that.

If you can find a switch that has good video quality and passthrough audio over HDMI, then you could use Arcana with that and probably be good.  Alternatively, you could maybe get some good HDMI splitters and/or switches so that you can watch sources without going through the multiview for the times you just want to show one source.  It can get complicated real quick.

Also worth noting that if you go with a projector usually at the back of the room, you would need a long HDMI cable for the Beam...still at the front of the room.

I might ask about this on a general AV form, like https://www.avsforum.com/, as folks there would have more experience with this issue.  HDMI-ARC and Sonos is really the easy part of this problem.  Getting good quality audio and video from a multiview switch is the harder part.