Here you go (UK Specific) - Plug the mains plug in to the same block as the amp / Sonos. Connect the RCA plug to a spare socket on the Sonos.
You only want to connect to the outer ring of the RCA connector. Leave the inner unconnected.
Connect to the earth pin only on the plug - this is a standard 3 pin UK mains plug. This will differ for you as you are outside the UK
As i said before, if you are still unsure, especially about the mains earth then seek local advice or as Chris suggests. add another source that does provide a signal earth to mains earth connection.