If you can find one, one of the old Sierra sound systems would be the one I would choose. Unless you run some sort of PWC for the motor control (DCC, Aristo Train engineer etc) , these are the simplest to fit and use.

I have just put the new Revolution Train Engineer into my Connie, along with a Sierra sound system. The controller is great, the sound works well. I dont like 'twist the knob' controllers for outdoor use, if you stay at the controller, you dont get to walk with the train or enjoy yhe garden. But if you do walk with the train then if something goes wrong you cant turn the power off quickly.

1.5Amps is plenty for that loco and carriage set, and your track would need to be a few hundred feet long before the power pack would be considered a problem. From your post, that could be a while away yet.

At the moment though, the market is a little bereft of a decent, low cost, sound system . Current pricing means you will need to spend about the same as the loco again for a sound system that is worth the effort.