The first sound system conversion I ever did was in the Aristo FA. I used this installation as my "test case" to see how easy it is to install a Sierra Soundtraxx system. For the FA, I chose the Alco with the Leslie F3 horn #831012.
The first step is to remove the fuel tank and open the body shell by removing all the screws in the underside. I decided to build a speaker box and push the sound out through the fan grill in the top of the body shell. I removed the fan, motor, smoke unit, and smoke tube to free up this area in the locomotive. All wiring for the smoke and fan were removed from the circuit board on the rear bulkhead of the locomotive.
The speaker box was built from acrylic approximately 1/8 inch thick. You could use styrene, but I had some scrap acrylic for my purposes. The box is glued together, and clear silicone is run around all the seams. It is approximately 3" square and about 2.5 inches deep. The top is cut out to fit the speaker and the speaker is siliconed into the hole cut in the top. I used a Phoenix speaker SP-2.5SQ which was the largest that would fit in the locomotive.