A few points, from a Bi-Polar owner who has taken them apart several times.
1. In a PS/2 engine, the battery is irrelevant to whether or not the sound comes on. If the unit runs, the sound should work. That said, your LHS is dead wrong about the battery. They go bad over time, either through memory effect or chemical deterioration. Usually even a degraded battery will be enough to keep a PS/2 locomotive operating, unlike a PS/1 unit which is much more dependent on a good battery. In fact, the battery in one of my Bi-Polars went bad a year or two ago; I replaced it with a BCR so I wouldn't have to open the thing up again.
2. Starting with the obvious, have you checked to make sure the volume control is turned all the way up? The volume control only functions in conventional mode (in DCS mode, the volume pot is out of the circuit and the handheld unit controls volume).
3. Another thing to check is the connection to the speaker, and the condition of the speaker itself. Early PS/2 speakers go bad with some regularity. You can usually spot a bum speaker by the fact that plating is flaking off the metal parts. Normally a bad speaker just sounds awful, but it could be completely blown. The good news is that speakers are cheap, easy to replace, and the replacements are reliable and sound better than the originals did when they were new.
4. Putting the unit on a DCS layout is a good idea. You can do a complete reset and see if the sound comes on. The fact that the lights came on after a non-DCS reset suggests that something is out of whack and a reset under DCS might help. Of course, to do that you have to get the thing back together, which as you've discovered, isn't all that easy.
5. I feel your pain with taking the thing apart. The segments can be detached without tools, if I remember correctly. You have to fiddle around with the pivot points, get the segments semi-apart, then unplug the wires connecting the ends to the middle. You are correct about the pans having to be attached from the inside. It's a pain, because it's possible to loosen them from the outside and then you have to take the thing apart to get them tight again. Be sure the nuts are nice and snug before reassembling the center segment.
6. The body not centering properly coming out of a curve may be caused by wires bunched up inside somewhere blocking the motion. I've found this to be pretty common on all kinds of diesel and electric locomotives.
7. You don't actually have to get the truck screws from MTH. I forget what size they are (probably M3), but if you have a big Ace Hardware or a decent hobby shop nearby, you should be able to take one of the screws in and match the threads. In the long run, a thread gauge is an excellent investment. Mine are called "Screw Check'r" and "Metric Screw Check'r." You'll probably have to cut off whatever screws you buy, but it's quicker than waiting for the parts to arrive from MTH.
8. Reconnecting the three segments is an obnoxious job. The trouble is in getting the plugs connected and the segments aligned at the same time. I usually use a large angle-head tweezers to handle the plugs. Other than that, I can't offer a lot of advice; I've done it entirely too many times, but the secret is in what the Germans call "Fingerspitzgefuhl," which one acquires through experience.
9. Did you get the instruction manual with the secondhand unit? If not, you should be able to download it from the MTH site. I seem to recall that it has some useful advice on disassembly and reassembly.
Good luck.
