Play multiple sounds at same time

My project is to build a kind of "electronic harp". There will be some sensors waiting for the the hand to pass by, and when it does a sound will be played. More than one "note" could be played at same time. I've seen several projects of electric harps, but most of them are plugged to midi keyboards, i want this to be independent (sounds will be fetched/stored in a SD card). I have some questions regarding this:

I'm using an Arduino Uno -

  1. For playing the sounds, I have bought ITEAD's mp3 shield ( http://blog.iteadstudio.com/itead-music-player-shield-plug-and-play/ ), which is the only one I could get here (I'm from Argentina, and here there are lots of limitations to bring products from the outside). The things is that I found some limitations using it. For instance, I can't play more than one sound at a time (because it mainly acts as a normal mp3 player). - So, maybe this is not the component I need to play the sounds.

  2. Which is the way to play two sounds at same time? I've heard of MIDI controllers, would this help me? (I dont want to depend on an electronic piano, nor a computer plugged in). Any other way (not shields, because hard to buy here, just standard electronic components).

Thanks in advance!

I don't know how feasible/practical this is, but in [u]this post[/u] I explain the concept of mixing sounds digitally by summing (or averaging) .

What I don't know how to do is, open multiple MP3s at the same time, decode/decompress the MP3 data, and then sum/average that data before sending the data to the DAC on your MP3 shield.