jsynth-midi
require it
pass it a midi file
it returns a function(time)
call that function over and over again, passing the time
it will sometimes return midi events
look for event.subType == noteOn || noteOff
in those objects you will find event.noteNumber and event.velocity properties
known bug
tempo changes may cause a problem, but so far only very complex midi files has those.
want to fix bug soon tho.