A clean earth is used to protect devices from noise from things like motors from fridges and vacuum cleaners, it will also protect the equipment from a surge on the earth during fault conditions from another item fed from the same distribution board.....
But...
They are almost always wired wrongly, you need a clean earth socket outlet that separates the earth to the pin of the sensitive kit from the socket pins and general earth in the building, a single cable should come from the fuseboard to the socket and the earth used for the kit, the other earth run along just connects the pins and back box etc straight back to the fuseboard. If you have a standard socket it isn't doing anything no matter what your ears say.
In a domestic setting with one fridge and one freezer I'd wager no difference in the audio system, as for 6mm over 2.5mm cable that's 100% nonsense.
Btw I'm a an electrician that wires these systems regularly, I've just finished two state of the art IT rooms at a school last week, they have dispensed with the clean earths other than to the network hubs, and the hubs feeding 70 stations are wired in 2.5mm with a 6mm clean earth, to a normal pointless socket outlet