Assuming the signal coming from the aux out on the Chromecast is an analog line level signal, sure. The Port has no knowledge about what device is connected the the line in, it just looks for that line level analog audio signal. 

Thanks for the reply!!

Actually it’s just the same as other music streaming services like Spotify, Apple Music etc. but just because those services are not integrated to Sonos but chromecast compatible. Do you mean logically it should be workable?

I have a Chromecast Audio lying around because the music services I use are integrated into either my Echo or Sonos so I don't need it anymore. But I know that it puts out analog audio that will be compatible with the line in on Sonos. I remember that the Chromecast has a setting somewhere that is to be used when it is connected to HiFi system input jacks, be sure to select that option.

By the way, Spotify is directly integrated into Sonos in most countries now. And both Spotify and Apple Music are available via the Sonos controller app.

Thanks Kumar for your kind and helpful advice!! :)

Sorry or I expressed not clear enough… I mean some other music services like Spotify or Apple Music, but cannot control from the app and not integrated to SONOS.

Spotify can be controlled from the Spotify App, Sonos speakers can be selected as targets in it. Apple Music can only be played from the Sonos App, you are correct. Amazon Music and You Tube is also integrated into Sonos. 

Anyway, if that does not suit you, wire a Chromecast Audio into the Aux In on Sonos and you are set.

Thanks! Yes and that’s what I would like to do… I have ordered my PORT and will have it with me tomorrow, then I can set it up. :)

It should work fine… I haven’t tried it into a Port, but currently have it working on the line-in to a Play5 G1. At one stage I did have it working into a ZP90 (Connect, earlier version of the Port), but decided to put the signal straight into the AV receiver instead (I can’t remember why, now).

As @Kumar  said, there are settings that are best changed for this type of use - IIRC, volume and quality.

Once it’s set up as the line in to the Port, you can use it as the source for other speakers - e.g. you can play the line in from the Port to another speaker, even whilst listening to something else on the Port - very clever, really.

Thank you so much and I will try! 

IIRC I needed a 3.5mm to 2xRCA converter for the ZP90, but I’m not sure what the Port has...