Not sure about the DAC, but running TV sound from the Beam to a Port will result in a (75ms?) delay. Since your speakers are close to your TV you will probably notice this delay. I'm also not sure if this "double stereo” effect you want to create would add anything to your TV experience. Maybe surround would add more? The Port is a good way to get modern thing like Spotify etc. to older devices though!

I use a Connnect (the Port's predecessor) the other way round: toi get sound from my CD-player into my Sonos environment.

Thanks for your reply. The idea is that I’d get the benefit of my larger speakers therefore adding to the tv audio. I thought if I ‘grouped’ the Sonos port with the beam speaker that it might work without a delay between the two. But from your answer you think that there would be a slight delay, is that correct? Thanks for your answer.

ps I also will be using my stereo connected to the port just for general audio listening via the Sonos app or airplay 2.

It will be fine for music, but as @106rallye says it won’t sync for TV.  Sonos was originally designed for multiroom synched music, and that is still fundamental to the system.  For that, you need a buffer so that the each bit (in computer sense) is ready to go on every speaker at the exact same instant.

As soon as you introduce video, the key requirement is acceptable lip sync so there is lower latency on the Beam when it is being used for TV sound through the input cable.  The result is an echo if you group with other Sonos speakers / players.

You could get round this (sort of) by delaying the audio on the Beam (there is a setting for that) but this would be at the cost of lip sync with the video.

I don’t think this is really going to enhance your TV audio experience.