I don't think you need to play 30 different sounds at once, unless you have 30 hands to play 30 different percussion pads!
I'm assuming you can play 2 sounds =2 hands at once.
You should additionally consider that when you hit a percussion the sound could overlap the tail of the sample that is currently played (if any).
I never wrotecode for DSPs, but that's probably what you need to do.
And this board could be the right one: http://www.openmusiclabs.com/2011/08/arduino-audio-codec-shield/
If you write good code, please share.
I'm building a drum kit that I will connect via MIDI to a PC for the generation of sounds.
If you find a good solution, my drum kit could become a stand alone device!