I know that this question has already been answered but I have found a simpler and cheaper way for anyone still struggling with this issue. 
 

I brought an audio extractor off  Amazon: 4K Audio Extractor HDMI by OREI

To make this all work, plug your source into the input and projector into the output, then plug the sound bar into the optical port. Make sure that in the projector settings the audio out device is set to AV receiver and the go to the sonos app and go though the “tv setup” after this you should be all good. 
At time of this the audio extractor is only £44 which is cheap compared to others on the market.

I hope this helps :)

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