More great advice in this thread! I would add to Scott's comments that you may be able to fit the next size up of Tang Band speaker - TB1931s - in the fuel tank if you cut the end tabs off. Well worth it for th extra 50Hz of bass imho. In my latest Atlas install a put the TB1931 facing up in the fuel tank and cut away the chassis above the speaker to let the air move.
Regarding decoders, I'm not sure if there's an add-on sound board that works with a D408SR. Back in the day I used Soundtraxx DSX I think. But sadly that combo won't run or sound anwywhere near as good as a LS5 DCC L decoder.
The starting off and slow running is hard to do well with the Atlas dual-motor drive. The D408SR does OK at this, but the LS5 DCC does a far better job of smoothing out the cheap can motors in these locos imho. Here's my first LS5 DCC in action in a consist after some tuning. Atlas C424 chassis on the left:
And here's the sound from the LS5 DCC. The C424 prime mover fires up first . Both locos have the TB1931s.
