Is there a (simple??) way of getting a sound to play through a pc? Maybe a .wav? If I can get this working then I'll be one step closer to my goal.
If you are using a PC, I'm not sure how you get the application running on the PC to interact with the Arduino... Maybe someone else can jump-in and advise on that.
If you can get the Arduino communicating with the PC, then the Arduino will be a good, inexpensive, way to "collect" the digital or analog from your optical sensors.
There are special-purpose cards/devices for connecting sensors (and relays, etc.) to a computer, and with something like that, you wouldn't need the Arduino. You should find this stuff if you look-up "data acquisition" or "data acquisition and control". These things come with drivers (and probably a software development kit) so getting the "information" into the computer fro use by yoru application is easier. The bad news that this stuff can be very expensive.
Since you are using a computer, it should be no problem to generate or play multiple sounds at the same time. As long as you know how (or you can learn how) to program in Visual Basic or Visual C++, and how to make your application communicate with the soundcard driver.
To do something like that without the computer, the Arduion by itself is only good for generating fairly simple square & rectangle wavforms. To play mp3, WAV or MIDI sounds, you'd need an additional audio "shield". And for multiple sounds, you'd probably need an Arduino & audio shield for each of your boxes.