2004 Mazda 6 Petrol - No Sound from Audio unit - Scarborough Pirate

Can anyone help. My CD was faulty so got a new unit with exactly the same inputs at the back however - on the new unit I get no sound. I notice it says 'Hello' on the display panel when turned on but my old unit said 'Bose Sound' Is there a wire needs switching or something. Any help appreciated.