dw, I have a Lionel FT diesel from a 2001 set, my railsounds were all scrambled up. I sent the loco to Charles Ro Supply, www.charlesro.com (the largest Lionel dealer in the US). They found some wires that had come loose (Lionel just taped them up) and rubbed on the inside of the loco from operating it. This in turn shorted out the hall sensor, thus scrambling the sounds, I still get sound but it was random. Your loco may have the same problem, you may have some wires inside shorting it out. There should be about 4 screws that you can access from under the loco (4 corners) and the cab should lift off. You may have to turn the front truck to access the front screws. Also beware, when you remove the cab, if there is a rear "ladder" on the loco, the ladder will fall off the frame, it mounts along with the screws.
(I am still waiting for the rear truck on my loco, Charles Ro has had it for 1 year! The whole truck must be replaced, as the hall sensor is pressed into it during the manufacturing process)
Hope this helps,
Post these problems in the Classic Toy Trains forum, and you will get more replies. As for Paul's problem, the TMCC antenna may not be getting a signal from the command base.