MIDI Synth program + Rasp Pi

Hey everyone. I'm using 5 Arduinos to connect to a Raspberry PI through 5 USB ports (I will use a hub since Pi has only 4 USB inputs). Arduinos have lasers and photodiodes, so basically a laser harp. I am looking for a virtual synthesizer program to run on Raspberry PI or Windows that will recognize 5 ports (each Arduino MIDI instrument) and play any types of sounds (piano, guitar, etc.)... Does anyone know of a program like that? Maybe something similar to Garage Band or Synthesia?

Thanks!