The best sound mod for the 1 speaker Sega's can be found here:
http://www.pinballnews.com/learn/speaker/index.html
Please note, the speaker suggestion is crap. I did this to both my SWT and X-files. Very happy with the results. I do need to play around with the sub a bit more to get some more boom - I should throw an L-pad into the mix as well.
If there are some critical pages from the manual, I'd be happy to scan a few, just PM me and remember, it's just me and a basic home scanner, so we won't scale to well to huge requests, but I'm happy to try.
The rubbery part of the file cabinet is pretty much just an overlay to the metal, so it seems (to me) an appropriate material for balls to bounce off of. Perhaps a mold could be made and them produced in silicon or other cheap bouncy material....?
Always happy to help out fellow x-files owners!
~Glenn