|"my Samsung Series 8 Led TV user manual does have the odd ref to USB ports but no ref to how to connect or what to use them for?"|
I downloaded the manual from Samsungs site and it's very specific about what the USB ports are for and what they can do.
It can be used with a flash drive with video and pictures on it.
It can be used with a USB mouse & keyboard
It doesn't mention audio out on the USB ports so I would say no it won't provide you with audio out.
I don't think an HDMI to USB adaptor will work.
You would be spending at least $50 - $100 for an optical to RCA or USB adaptor. And since the Samsung does not output audio through the USB port, no it won't work.
Do you have the Control Pod for your Companion 5 speakers? if so, it has an extra analog input. Just run the audio output from your Samsung into there.