I just got an HO GP40 Diesel Loco- DCC Equipped Union Pacific and it runs great.  It has lights and will run on DC or DCC.  Can Sound be added to these already DCC equipped Locomotives from Bachmann or must the current decoder for DCC be switched out?  How if so?  I'd love to hear from anyone that has added sound to one of the Bachmann DCC equipped locos since I own three of them now and none of them have sound, only lights.

On a separate issue I'm building my tabletop layout and was wondering what people are using to wire their DCC layouts...I have 16 gauge and 18 gauge wire at the moment and was thinking of using the 18 to distribute power around the layout with the 16 serving as feeders to track sections.  Is DCC wiring a lot different from DC since you don't need power districts?  I don't notice much drop in train speeds on my layout...do I even need feeders and a bus?  My layout is 4 x 7 and consists of a pasing lane and odd shaped oval with an internal oval running seperately on DC.  Any thoughts and comments would be appreciated .