I really would like an RS-3 with sound, but not if I have to resort to MRC decoders. It will be hard to fit anything but the absolute tiniest speaker in there withough hacking away some of the weight somewhere. I did hit on what seems like a possible idea though, I have 4 Atlas undec ones to do, plus I have a 5th one that has a basd motor. Replacement motors are still available from Atlas, but that would cost more than I paid for the loco (which did have a working, if noisy, motor - it’s ‘broken’ because when I disassembled it to rey and fix it I lost a brush spring). SO I thought of perhaps removing the gears and weights and making a sound dummy out of it. Not an enormous amount of room, but a long rectangular speaker enclosure should fit nicely.
I haven’t attempted to add sound to an Athearn RS-3, but when I was researching the electrical pick-up issue you mentioned, I came across a tutorial on how to do just what you are looking to do, as well as fix the pick-up issue. The tutorial is on a Yahoo forum (Rick Bell Clinics) here: http://groups.yahoo.com/group/RPMclinics-RickBell/files/
In the “file” area you’ll see the listed as a pdf -" Athearn RS3 Sound Install.pdf Athearn RS3 Sound Install Two Speakers "
It looks like he used the following in his install.
Mini Oval Speaker
Small Oval Speaker
Soundtraxx TSU AT 1000 #828043 Alco 244 sound decoder