I have done this before in my Silver SD35s but I wanted to report that I just popped in a Loksound decoder into one Silver C420 and one Silver SD35 (the recent ones with the factory speaker).
I was reminded, pleasantly, how well Atlas has engineered the speaker and enclosures. Both sound great. No tinny horn or thin engine sounds.
My N benchmark for excellent sound are my first-run Rapido Dash 8-40CMs with factory sound. They had robust, full-throated horns and excellent prime-mover sounds. These Atlas engines sound just as nice.
I will put my obligatory ScaleTrains comment in brackets: [I wish ScaleTrains would figure out how to enclose their speakers properly.] BTW, I own 12 of their rivet counter engines.
So, thanks Atlas for an easy upgrade to sound. Easy shell removal and easy decoder installation!