Shouldn't be hard for the right sort of techie. The sound cues for aiming a satellite dish would be a good model. This is not a real high tech operation with the current generation of small dishes. You start by pointing it to the prescribed compass heading and angle from vertical, according to your location, then turn on the TV & satellite dish using the "aim dish" feature, and turn the TV volume up. When the dish is not locked into a signal, there's a low-pitched broken sound, when it locks in it goes to a higher pitch without beeping, and the closer it is to 100% signal strength the higher it gets. You just keep "tuning" the dish position by moving it a tiny bit at a time, until the pitch starts to go down again. The analogy is obvious.   

Of course it seems a little odd to use a sound-based system to tune a sound, but its easier to discern because the signal tones are prescribed "steps" instead of always changing in an analog fashion. One must tune to 440 or whatever in order to be a sociable musician. One refinement you could make on the satellite system is to have the signal solid when the tone is flat, and beeping when it's sharp. (Danger, danger Will Robinson!)

I guess if no one has made it already, it's because the market is perceived as too small, since the perception of blind musicians is that they have such great hearing that they wouldn't need a tuner!

Gee, there are software tuners on the market, a programmer should easily be able to modify one to emit sounds.

W-O