A few months ago I imported FireRed instruments into Ruby. This, however, lets you import any instrument... The only problem for me is how on earth do you get the WAV file for an instrument?
Well, you can rip sounds from other games using the "Record Sound" feature (This is where I recommend using Audacity to edit the sound clip), or you could try some sound packs for Fruity Loops Studio. There's tons of them out there and most are in WAV format. In fact, that's how I got the sample sound for the "Custom Instrument" video.