LIONEL SD70ACe SOUND BOARD FIXED?

Nice fix.  Yes on some of the Lionel Diesels you need to pull the shell outward with fingers as you slide it down.  They are tight fits.  I am sure to maintain that 1/48 scale accuracy.  G

I have this engine in the NW Heritage scheme, and am having the same issue.  Sometimes the sound would not work when starting the engine, other times it would just stop while the engine was running.  I determined this is a design flaw in the electronics.  There are at least 2 boards that plug in to the main motherboard, one of which is the sound module.  It sits on top of a long riser pin set coming up off the main board.  It has an oversize capacitor on it that hits the small circuit board below it, thus preventing it from seating properly and/or fully on the riser pins.  On top of that, like you stated, the shell is very close and tight to the electronic, it really takes some precision finagling to get it back on properly.  My theory is that because the sound board doesn't seat properly, that over time when the engine is running, the vibrations cause it to gradually work loose and become unseated enough to disrupt the connections and then you lose the sound.  When this happens if you tap on the top of the engine a few times it tends to come back, unless it has worked too loose, then the shell has to be removed and the board reseated.  It's a pain to be sure.  The proper solution would be the replacement of said capacitor with a shorter one the doesn't hit the lower board.  I may try the fishing line to see if that helps hold things together as it's cheap and easy.