The next time it happens, run a system diagnostic and call Sonos Support directly to discuss it. They should be able to determine where the command for raising the volume is coming from. Traditionally, this has been one of several things….a stuck button on a physical device, such as a phone, or something sitting on the speaker itself against the buttons there. Or even sometimes, if you’re a Spotify user, from a remote person using Spotify Connect to reach your system and turn things up by mistake (this “connection” can be cleared by playing anything that is not Spotify on your Sonos).

Let us know what you find out. Especially if it’s a software issue, but in the handful of posts I’ve seen about this kind of thing, there’s always been some other reason for it. 

I have had a similar situation with (only) the Play:1 in the bedroom.
Have gone to bed with the volume down to “3” (that’s the lowest
before you can’t hear anything (?) - if you can believe it) and during
the two hours that it’s on the timer, it has started climbing in volume
in the middle of the night making me sit bolt upright in bed and scared
to death. It did that last night again while my wife was sleeping and
I was temporarily in the bathroom. There’s just the two of us here.

And the same reply applies. 

Really.

I wonder about that “remote person” turning it up in the middle of the night or
that phantom object sitting on the volume button. I can’t say that I’ve ever entertained
the idea of being haunted. Oh, and as for running a diagnostic in the middle of the night?

Nah.

There have been cases of animals sitting on the devices triggering the buttons, remotes ‘stuck’ under cushions, buttons on phones being ‘stuck’, etc. So far, it has never been an issue with Sonos’ software itself, in any of the cases. Unless you’re willing to provide hard data to Sonos, there will never be proof one way or the other.

Well, if there were any animals in my bedroom, I’d probably notice them.
I don’t remember the speaker coming with a remote and since I don’t have a cell phone,
I’m not familiar with how a speaker connects with a phone.

Anyway, I’m not trying to put blame on Sonos, so I don’t know why you’re getting all defensive,
but was just saying that I have had a similar, unexplained situation happen here on a single speaker in the middle of the night, but, you know, it’s definitely not software!!!!!!

Gosh, now I’m afraid to ask why the sound suddenly cuts off after you go below “3” in volume.
I thought maybe it was because of having the loudness switched on, but I don’t.
Maybe I’ll hunt for an answer somewhere else from people less triggered...

Making a diagnostic without a cell phone might prove impossible.

I had the same issue with our brand new Ray randomly turning the volume to max quickly. No one else was connected by phone, and there was nothing in front of or touching the touch controls, however when I turned out the touch controls, the issue went away. 

Seems that there is an issue with the touch control. This unit is on day 3, so not thrilled about that.

Turn the controls back on. wait for the problem, submit a diagnostic, call Sonos with the number it provides and get the Ray replaced if defective.