Without at least some info on the Beocenter 2300 (not many people have the specs of it at hand) it will be hard to help you. So a questions first does the Beocenter have an output for audio? Sometimes it will say “tape out” or “Zone 2” or such.

If you have an output on that device you’d likely want a Stereo RCA male to Stereo 3.5 mm male cable.

Plug it into either Five and select the Line-In to listen to it.

Thank you for the answers folk.

Turn out it is quite easier then I thought. Just use the line in and set the pairs of Five on stereo pair. However I needed to add a ground isolator block to stop the buzzing noise. Works like a champ now !

So the Beocenter has a line out?

Yup..beocenter 2300 has line out at bottom right corner. You only need 1 line in to one of the Sonos if they are stereo pair.

You might try flipping one power plug, see if the hum stops. If not flip it back and try flipping another. Try flipping them all - BUT - only one at a time. That will find any single problem device that can be cured this way.