Sonos does not publish roadmaps.  They do have an API for volume normalization, but it is up to the music service to access that portion of the API in order to implement it. 

Indeed. See replayGain on https://developer.sonos.com/reference/types/playback-objects

Note that Spotify do not appear to ever return this data.

Do you know @controlav if any of the major music services in Sonos use this normalisation feature, as I’ve not (yet) seen an option to toggle it off/on in the App and I would personally want the option to leave it ‘disabled’ and hear all tracks as intended. 

Confession: I wasn’t aware of this feature until jgatie pointed it out above (Sonos long gave up on change notes for their developer pages).

However, I am not so sure this feature really exists. This sole reference in the docs is for the cloud API, not SMAPI (which is what the speakers use). SMAPI makes no reference to this at all:. https://developer.sonos.com/reference/types/smapi-object-types/#trackMetadata

Now it is possible the SMAPI docs are out of date, and jgatie clearly knows more than I in this space. It is also possible that the warning at the top of the cloud docs that not everything is necessarily implemented may be true for this attribute, in which case there is no underlying normalization feature (yet).

Thanks @controlav, appreciate the reply.

Adding another voice of support for the need to allow normalization of audio coming from Spotify. There are countless threads in this support forum with people requesting this feature and explaining why it’s so frustrating not to have it.

What do you mean by Normalization?

Spotify like all streaming platforms perform normalisation when studios/labels/artists upload the source music to the platform. The perform normalisation server side in two instances. Per album normalisation (some platforms only perform per track ) and when creating a shuffled album or playlist.

https://support.spotify.com/us/artists/article/loudness-normalization/

Note: The web player and 3rd-party devices (e.g. speakers and TVs) don’t use loudness normalization.

In the Spotify client normalisation appears slightly different and adjusts volume based on 3 presets, so more like dynamic range control or night mode style adjustment from my reading.

https://support.spotify.com/uk/article/audio-normalization/ 

So normalisation in terms of replay gain during playback doesn’t appear to be anything Spotify provide or expect devices to attempt to do. Sonos would have a hard time trying to sensibly normalise even per-track unless they downloaded and analysed it before playback.