I haven't done a conversion with any Atlas Loco.

However, it is really a pretty straight forward job to install any decoder in a non decoder equipped engine.  The biggest challenge with an engine that was not designed for a decoder is to get everything properly insulated and find space for the decoder and in this case speakers.  You want the motor brushes totally isolated from the electrical pick up.  Your decoder should have come with an instruction sheet.  Just follow the instructions.  In your case, with the frame that Atlas designed for the factory "Gold" units, the space issue has been solved.  To install the speakers in parrallel, you just take one wire from each speaker, wire them together, and then wire them to one of the speaker wires from the decoder.  The other speaker wire from the decoder needs to be wired to the two resistors, then each resistor is wired to the left over speaker wire from each speaker.  To insulate the wires between the speakers and the decoder from shorting to the frame, pick up some heat shrink tubing just big enough to slip over the resistors.  You cut the heat shrink long enough to cover the resistor and any bare wire, but short enough to move it out of the way while soldering the wires together.  Once the solder joint is made and has cooled, slide the heat shrink down over the resistors and bare wires, and heat with a small torch, throw away lighter or even a match.  If you don't have enough room to use the heat shrink, you can wrap the connection with electrical tape, but that is not as permanent a solution.