There is a standard mapping for percussion instruments in the General
MIDI specification, and it is very inclusive.  You can find it here:
http://www.midi.org/techspecs/gm1sound.php  (scroll down to "Percussion
Key Map").

All electronic instruments, modules, sound cards, and software synths
that conform to the "General MIDI" specification _must_ follow these
mappings.  Most, but not all, Standard MIDI Files posted to the
Internet follow them.  So if you get your system to respond to the
individual notes the same way, you should have pretty good luck with
downloaded files.

Paul Lehrman