correct mount of era 300?

Seems odd to hang it from the ceiling, but unless there is something pressing on the buttons, it shouldn’t change volume on its own. I’d try a couple of things. First, disable the ‘touch’ buttons on it, by going into the controller and turning off ‘touch controls’. If that doesn’t resolve it, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here. They should be able to see in that diagnostic where the volume change command came from, I think. 

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

The Era-300 does have an orientation sensor, but I'm not sure if it shuts down the volume if it detects an unsupported orientation.  Even so, that mounting position is decidedly less than optimal.

I don’t know what your seating arrangement is; but if you need a ceiling mount for Era 300’s click the link. Not a personal endorsement but; Flexson is a respected company for Sonos stands and mounts. 
 

https://www.amazon.com/FLEXSON-Ceiling-Mount-Black-Single/dp/B0CHK1PXNY/ref=mp_s_a_1_1

Thanks everyone. Seems like ceiling mount is not optimal but wall mount isn’t really feasible so going with the flexson ceiling mount!