MTH 2-10-0 decapod with weak sounds and no manual

The manual should be very similar to other similar vintage locomotives with the 3/2T switching, so most of the info you need should be in the other manuals.  Obviously, you can contact MTH and ask about obtaining a manual.

You might be able to improve the sounds by changing the baffle on the speaker, otherwise I'd have to just search around for a better speaker that would fit.  I buy speakers from a variety of sources for my repairs and upgrades.

I would open the tender and inspect the speaker.  There are several causes of low volume.  One is a defective speaker, or one of the wrong rating.  Yours is a PS-2 3V engine so the speaker should be 4 ohms.  A proper MTH speaker will be plenty loud.  Maybe someone put an 8 or 16 ohms speaker.

Second is a bad battery or BCR that drags down the power supply.  Probably not in your case since the battery test was normal.

Last is a defective processor board.

 G

MTH had an issue withs speakers failing a few years back.  The magnet would come apart.  A replacement is cheap and easy to replace. Might want to check out the speaker.  

MTH had an issue withs speakers failing a few years back.  The magnet would come apart.  A replacement is cheap and easy to replace. Might want to check out the speaker.  

That was an issue with some 5 volt PS2 only.