Maybe see if you also switch off Automatically "update iOS Apps" in your iDevice settings, perhaps puts a stop to the notification you’re seeing in the Sonos App.

See screenshot (just as an example, taken from an iPad). 

Hey Ken, 

It’s not the app update thats the problem, it’s the update for the Sonos System within the app. I don’t have a problem with being alerted, but its that I am then reminded and encouraged to enable automatic updates, with no “update now” button to be found without going back through the settings. If the app has told me there is an update, I should be able to tap that alert at the top of the screen and update straight away. 

My thoughts were, that you would not get any update messages at all in the App, if you disable both the Sonos and iPhone ‘automatic’ update features and so when you became aware that a new update was available (via a manual check) you could (a) research that update and (b) install it as necessary from "Settings/System/System Updates" with no nagging and it would leave you to choose when to check and when to update all from the same spot in the App.