Have you checked with Spotify? Sonos doesn’t have access to Spotify’s code base. 

I’m using iOS 17.4 on both iPad and iPhone and use the Spotify ‘free’ version. I just quickly tested this and the Spotify app volume slider and the Sonos App volume slider are both working here when playing to a Sonos Move speaker (called Portable in my system). That’s playback from their App using Spotify ‘direct control’. A screenshot is attached, but just to say that this was just a quick initial test.

I have the same Problem since IOS 17.4.

Please see below….

1. On the left of the screen is the Sonos App and the ‘now playing’ screen showing the volume control/level of a Sonos Move called ‘Portable’
2. On the right of the screen is the Spotify App using a ‘direct control’ link to ‘Portable’ 

I just simply adjust the volume control for ‘Portable’ in the Spotify App and as you can (hopefully) see the volume level of that speaker is adjusting and the Sonos App also reflects the change in volume.

This is on an iPad running iOS 17.4 - I’ve just used this device to demonstrate it is working, but if you’re experience is different, then I would reboot your router and devices, including speaker(s) and mobile …and if the issue persists after that, then perhaps get in touch with Spotify support staff and supply them with the information required, perhaps include a video of your issue too.

I have the same problem. Also there is a 3-5 second delay between play/pause from when I click the buttons 

Such a delay can often be down to the mobile device communicating with your Sonos speaker. Maybe see what happens when both are using the same WiFi band and wireless network access point.

Also, if relevant, close any other Apps on the mobile and check that any security software like VPN, Firewall, Antivirus, WiFi Calling, Dial Assist, Private network address (MAC address spoofing), or mobile data access by the Sonos App is perhaps not getting in the way of that communication.

In addition to that excellent advice, it also may be worth reading the wifi interference FAQ. 

Same problem here on iPhone 14 Pro Mac on latest iOS. Volume slider is not working on either Spotify or in the Sonos app.  We tried on different devices and non is working.  
 

So you suggest to reinstall our system? It is always such a hassle and takes long time with all devices. 

Do NOT factory reset anything, that will be a total waste of your time.

I am also skeptical that an app re-install will do anything for this issue.

Have you tried my iOS app (“Phonos Plus”)? If its volume control doesn’t work either than that is an iOS problem as I share no code with Spotify or Sonos apps.

Yes - I also have this issue. Not sure if it was iOS upgrade or new Sonos app. Any device with Spotify connect is no longer working with Spotify volume. The volume in the Sonos app still works, but I only use that for config. iPhone 15 Pro, 2xSonos Move, Arc, Sub3, 2xOne SL. Will try with an iPhone 13 Pro not yet upgrade to latest iOS to see if can still do volume control

same here with my iPhone 15 pro. Restarting my phone temporarily fixes the issue but I’ve resorted to using airplay instead of the direct to Sonos connection.

Sonos Spotify connect devices are no longer able to adjust volume with volume buttons in the Spotify app after upgrading to iOS 17.5.1. The volume in the Sonos app still works and when using airplay they also works.

I have tested it with 3 phones and this fully updated Sonos setup

1) 2xSonos Move (stereo pair)

2) Arc, Sub3, 2xOne SL.

On both Sonos systems the volume control stopped working in Spotify app after upgrade from iOS 17.4.x to 17.5.x. Sonos App OS is S2 build 80.00.08, Model 48. One/Sub/Move are version 16.2, Arc is 16.1.1

I’ve tested it on these phones:

Does not work on IPhone 15 Pro iOS 17.5.1. Spotify 8.9.40

Works on IPhone 13 Pro iOS 17.4.1. Spotify 8.9.40

Works on IPhone 11 Pro iOS 17.4.1. Spotify 8.9.40

I have the same issue if I connect to the system directly, but if I use AirPlay to connect, it lets me control the volume.

I had the same issue, but now I think it is intentional, let me explain:

I have always used Iphone 12 side buttons to control volume when playing on Sonos 3 Play via Spotify app. But this had its own issues when also browsing in social platfroms like Insta, X, the button didnt know always which media volume to adjust. 
 

iPhone volume buttons no longer do anything for me to control Sonos volume when playing via Spotify. But the manual volume slider showing up in Spotify device selection screen works!

The volume controls on an iOS controller device will work if using AirPlay, as the audio sent to the Sonos speaker, or group, is ‘playing’ through the mobile/player App. 

The Sonos App is not an audio ‘player’, it’s a ‘remote’ and the audio initiated using that App, is sent over wireless direct from the chosen source direct to the speaker, or group. It does not play through the iOS mobile device and so in that situation the mobile device volume controls cannot be used to control the volume.

You can use the Sonos App though, or the hardware buttons on the speakers themselves to adjust volume levels. I tend to change speaker/group volume using a voice assistant, either Alexa, or SVC, in my case.