[tacman7]

I'm not aware of any software that does what you want specifically, it may be out there though.

What you can do is get a sampler (there is a great free one called independence).

That's what samplers do, they assign sounds to keys. They also repitch the sounds so a wav of a violin playing a c note would be pitched up a half step and assigned to the c# key and down a half step and assigned to the b key.

Many programs like cubaseSX3 can convert mp3's to wav during the import process.

Maybe you can get a sampler that does that also or a stand alone conversion program.

I have a friend who used to do live theater with his computer using sound fonts. Good if you have any sort of sound blaster card.