First, I'd grab a pair of big old headphones off eBay. Get 6 mics (3 for each side), and some other things.
One mic on each side is normal hearing frequencies. Ignore that one for now.
One on each side picks up frequencies below my hearing range, and runs them through something that somehow increases the pitch without changing speed. (I know such a thing exists, just dunno how to do that yet. Any ideas?)
One on each side picks up frequencies above my hearing range, and runs them through something that somehow decreases the pitch without stretching it out slower. (Yet again, I know such a thing exists, just dunno how to do that yet. Any ideas?)
Then the 3 resulting outputs are merged into one and played into that side of the headphones.
Yes, I could cut the parts in half simply by using 3 centered mics instead of 3 on each side, but I'd like to try to enhance the stereo effect instead of replacing it with mono...
(Hmm. I bet I could do the same with video, too, just upshift IR to red spectrum, downshift UV to violet/indigo spectrum, and superimpose over normal vision by projecting it onto my glasses lenses or something...)