Are you saying it sounds louder from one source to another? Or that the volume slider keeps changing? 
 

If the former, you need to ask all of the service providers to agree upon a standard signal level. If the latter, it may be best to generate a diagnostics report and contact Sonos support. 

It's the latter, ie the sound level keeps changing 

Also worthy checking if someone’s on your Wi-Fi network playing with the settings on the app. 

Im having a similar issue. Its almost as if its switching between “night sound” and regular randomly throughout the day. Sometimes it happens multiple times in an hour, and sometimes nothing for days. 

I think I’ve been having the exact same problem the last month. 

Volume jumps every now and then within the hour, between a quieter volume to what seems a clearer higher volume.