Do you mean that you've chipped it with a non-sound decoder, and want to get rid of the toy-like chuffing from the basic factory sound unit? If it's a fairly modern Stainz with the built-in sound, the unit connects via a 2-pin plug (with 2 red wires) in the back of the secondary circuit board housed in the firebox, where the 3-way switch is mounted. Pull this plug (you'll have to dismantle the loco again to get at it, unfortunately), and you've disconnected the sound unit.
Jon.