Has anyone tried to put a sound decoder in a Bachmann DCC equipped GP 40 or an SD 40-2?
I haven't but I have looked inside them, the route would be to put the sound in a non-powered unit or in a trailing piece of rolling stock that you would run with them.
I think the below is your locomotive. From what I see, the motor is isolated and the loco has 12 volt bulbs.
http://www.bachmanntrains.com/home-usa/dwg/dwgs/H635X_IS005.pdf
I would go with a Tsunami and 16mm x 35mm speaker in a home made baffle. I just have no idea on how the sound would be. The diesel installs I did had the speaker over the truck on one end that fires the sound down becasue the frame over the truck is open. Yours seems to be solid chassis.
There are the shrink wrapped Tsunami and light board Tsunami.
Go look at the SoundTraxx site.
http://www.soundtraxx.com/choose/step3.php?t=ho&s=Bachmann
Also, contact Litchfield and ask. They have helped me. Very good prices and shipping. Decoder come with full instructions.
http://www.litchfieldstation.com/xcart/home.php
If the Tsunami is to expensive, ask about the Digitrax Sound bug. I know nothing about this decoder.
http://www.litchfieldstation.com/xcart/home.php?cat=268
Thi is not plug and play so you will have to do some soldering.
Rich