https://support.sonos.com/en-us/article/release-notes-for-sonos-s2

Sonos has removed this functionality from the app altogether.

https://support.sonos.com/en-ca/article/release-notes-for-sonos-s2

pEdit] -- just saw the reply above. I’m not thrilled about the change -- this is a pretty big reduction in the usability of the Sonos ecosystem.

In order to provide that Lock Screen Volume control SONOS had to use a hack because the SONOS speakers are playing the music not the phone/pad. I know that there was a celebration when SONOS developed that hack, but Apple has made continuing use of that hack impractical. If your use focuses on the Volume controls, and your SONOS units support AirPlay, you can send music to the SONOS system using AirPlay and use the AirPlay Volume control.

I’m not a regular AirPlay user, but in my limited experience with AirPlay, it is not the most reliable way to play music. And, once SONOS is playing the music, I don’t need to waste phone/pad battery charge while playing music.

I have the latest IOS and the latest Sonos software and my Lock Screen controls are all working. Now this is a good and bad thing. I have always hated how when I’m listening to a YouTube video or anything video wise with audio and I turn the volume up to hear it the Sonos app goes to that audio lvl when the phone goes to the Lock Screen. I can hit 90% volume on the Sonos as it don’t know any different. How can I turn this damn feature off as hardware volume control is no longer there yet my phone still does it. I can change track still too. This is a brand new iPhone with new iOS and latest Sonos software. 

This is an iOS Airplay issue - but maybe see if this link solves the issue for you (it’s only available though for linked devices):

https://www.howtogeek.com/730983/how-to-hide-iphone-lock-screen-playback-controls-for-airplay/

Removing the ability to control the volume on Sonos hardware with the smartphone app is core functionality and the exclusion of that capability is completely unacceptable. I can ALMOST understand the hardware and lock screen inability to control the volume if it doesn’t comply with Apple’s standards, (although it seems like an engineering fix could remedy this), but to leave your customers hanging without the ability to control your hardware volume, say through a slider in the app interface, is simply a “we don’t care about you or your future business” stance. I’m moving on to another platform. This is total BS.

Apple made this change to every company who tried to use a similar system to use the hardware controls, they didn’t single out Sonos. It’s just that there aren’t many other companies that use a ‘app runs on the speakers, not the iOS device’ paradigm. It’s unfortunate that Apple made this choice, and forced Sonos to follow along. 

Yes, the Apple hardware controls now only work if the audio stream is passing through the iOS device (so it will work with Bluetooth and Airplay playing Apps).

The Sonos App on the other hand is just a ‘remote’ and no audio goes via the iOS device - the Sonos speakers get their audio direct from the LAN/WAN source - nothing goes via the controller device.

I agree though Apple really needs to change this policy and perhaps find a secure way for ‘remote’ Apps to control volume and use the iOS hardware buttons/lock-screen controls etc; instead of confining their services to App audio-players only.