I'm by no means an expert, but have probably done somewhere between one and two dozen DCC installs on my locomotive fleet. I've used NCE, Digitrax, ESU, Soundtraxx, and I think even one TCS decoder, both with sound and without.
When it comes down to sound decoders, I like both ESU and Soundtraxx. However, at this point I've settled primarily on Soundtraxx. ESU are good, but I just find them a little quirky compared to Soundtraxx. I also seem to recall...my memory may be a little off here, but I think my experience with ESU speakers was that they are a little more sensitive to installation (e.g. baffles, sound box isolation, etc.) than Soundtraxx speakers in producing the best sound.
I know this is your first DCC project, and don't know what kind of electronics or soldering experience you have, but if it's not too complicated under the shell, you could consider simply replacing the entire Atlas electronics board with a sound decoder board (such as a TSU-PNP8). On some older locomotives, I've found that running a sound decoder through a plug-in to the original circuit board can result in temperamental performance sometimes. But there are times when I'll use a plug-in, such as when the original board has complex light wiring. I recently installed a sound decoder in an Athearn Genesis U50. Removing and reconnecting the headlight wiring would have required me to break off and re-install the glued-in cab interior; since I'm inherently lazy, I just used a plug-in decoder, preserving the original board and lighting attachments.
Good luck to you.