The question of which sound decoder pops up every so often here, so here is the link again for those who missed it before.
You have to figure out the decoder and speaker install.
There are Bachmann specific locos mentioned, including some diesels.
http://www.soundtraxx.com/choose/step3.php?t=ho&s=Bachmann
SoundTraxx has suggestions for some N and O, On3. On30 scale locos Just click on Decoder Selector.
Store the link in Favorites.
Rich