Hi

The Sonos Port is typically used as follows:

1. Audio received by Port from a connected source such as a CD player or Turntable via Ports RCA-In is broadcast to a select Sonos speaker via Line-In option in the Sonos app. Other Sonos can play the same music via process called Grouping to the select speaker.
2. Port is connected to an AV system RCA-Out via Ports RCA-In and music from a source concerted to the AV system is sent to other Sonos speakers via Grouping. If the AV system has speakers wired to it and the receiving Sonos speaker is in the same room there will be a slight delay to Sonos creating an echo effect. If the Sonos speaker is elsewhere the slight delay will most likely not be noticed
3. Port is connected to an AV system RCA-In via Port RCA-Out to send streamed audio to AV system wired speakers 
4. Any TV audio sent to a Sonos speaker/or component not part of a Sonos home theater setup (i.e. Sonos soundbar with or without sub/surrounds) will experience a lip-sync issue

And more specifically, Google has chosen not to integrate Chromecast with the Sonos software, like Apple has done with AirPlay 2.
 

You could ask them why, perhaps, many of us have theories, none of which have been confirmed by Google. 

You’ve got answers already, but let me try a different way.  It sounds like you intend to keep your existing projector and hifi system in place, and that’s fine. You can still use this setup as is, with chromecast, etc.  

A Sonos port can send audio to your hifi, for streaming audio or any other audio coming from your Sonos system. You won’t hear a delay with this, unless it’s added by your hifi.   It also can take input from your hifi, assuming your hifi can output audio via RCA (some receivers can, others cannot). The audio it takes in will be delayed by around 75ms.  That will probably be an issue if you have Sonos speakers in the same room as your hifi, but if they are in other rooms of the house, you likely won’t notice.

As others stated, you can’t use chromecast to send audio directly to the Port, but you really won’t need to if you’re keeping your existing setup.

Thanks for the answers.

I didn't specify it but the main reason for using the Port would be precisely to use it to integrate the existing system into the multiroom system and to connect a turntable.

I was just wondering if it would be possible to connect the projector, apparently not.

I currently use a cheap bluethoot receiver for audio from the projector, but it uses the last available input in the amplifier, which I should need to connect the Port 

Thanks anyway :)

Unless your BT receiver is using aptX Low Latency (or perhaps the latest aptX Adaptive) you’ll already be incurring a delay greater than Sonos’ 75ms Line-In default.