if i remeber well i think that you have to work on the Program Change on the sp505 in order to make sounds the midi notes on 17>32 parts...i've tried this months ago, so i don't remeer if this thing works...

man i wish i could help you out but ive long sold my 505 and mpd. i had em both at one point and tried to do the same thing youre doin tryin to sync em up but it just frustrated the hell outta me. i think someone here actually succeeded making them both work.

basical thing: have you set the same midi channel? then try to work with program change, i'm sure it works (i've tried the sp505 with the an200 and it worked that way)