With the AudioFileProcessor you can only play one sample at a time. In this case you need to create a soundfont and use it with the sf2 player. To create a soundfont you use a soundfont editor.
I've used this one and it takes some effort to get started with.
https://sourceforge.net/projects/swami/
Here is a new one that I haven't tried yet:
https://sourceforge.net/projects/polyph ... eases/1.0/