Help me Sonos Community, your my only hope!

I am having trouble setting up a Home Theatre at my dad's house and I'm needing some assistance.

At my own home I'm having no issues. I have my Sonos Playbar (Original) hooked up to my TV via the Digital Optical Cable but when I want to use my Benq w1070 Projector with an Amazon Firestick plugged into the HDMI port I hook it the Projector up to my Sonos Port via RCA Audio Cable and feed the sound to the Sonos Playbar via Line-In on the Sonos App.

I've attempted the same set up at my Dad's using his Sonos Beam and his Benq w1050 Projector with an Amazon Firestick plugged in to the HDMI port but it's not working. The sound instead continues to come through the Projectors internal Speaker. We managed to get it to work one evening but after turning off the system and coming back the next day the sound was back to coming through the Benq w1050's own internal speaker.

How can I get this to work or is there a better way?

Any help would be greatly appreciated.