Note the Sonos Amp requires an input from an HDMI connection with an Audio Return Channel (ARC) not a straightforward HDMI connection, so you will need to connect a device with HDMI-ARC.

Otherwise you will need to use the optical connection with the Optical-to-HDMI adapter (By the way I don’t think the adapter is supplied with the Sonos Amp (not sure?), but it is certainly available to purchase from the Sonos website, if required).. oh and I don’t believe that the available sound quality is reduced by using the Optical adapter compared to the HDMI-ARC connection, but you will lose HDMI-CEC control (obviously).

I have a possible solution but I need to know the brand/model of your Blu-ray player.

Note that for the same codecs, there is no reduction in sound quality when carried over optical or HDMI. The digital signal is the same, regardless of the carrier.  If you’re using Dolby Digital, there’s no difference. The difference occurs when you use “higher” codecs which are too large a bandwidth to be sent across optical, but there’s no degradation. It either passes through, or it doesn’t. 

Hi, Its the Sony UBP=X700

https://www.sony.com.au/image/5f2091a3a4e27ac6c6b2ea1a58ad22dc?fmt=png-alpha&wid=1440

I have a few Blu-Ray Audio only discs that have DTS Master Audio or Multichennel PCM and have surround and subs attached. Its my understanding that optical can not pass these through at highest quality

The Sonos Amp only supports stereo PCM, Dolby Digital, and DTS Digital Surround audio. So you will not be able to hear lossless DTS-HD Master Audio or LPCM from those Blu-ray discs. Only the Arc and Beam (Gen 2) support lossless audio formats. DTS-HD Master Audio is not supported on any Sonos device.

Ok, so basically using the Optical cable gives me the maximum a Sonos Amp can do anyway?

Yes.