You haven’t been able to do that for well over a year due to Apple’s rules.  See this thread:

https://en.community.sonos.com/controllers%2Dand%2Dmusic%2Dservices%2D229131/update%2D15%2D6%2Dno%2Dvolume%2Dcontrol%2D6884379

Thank you for the prompt response, that’s too bad, I hope they can work something out as I think is a key feature 

One ‘way’ around is to use either Bluetooth or AirPlay 2 on your phone, to connect to Sonos devices that can receive that type of signal. That way, the music generation is occurring on the phone directly, so Apple will allow it to have access to the button controls.