This is an Apple PlainTalk microphone and boy, they were great. I had one that was attached to my old Performa's monitor, which kind of perched near the center-top of the bezel. They were supposed to be used for various speech recognition things and software that came with OS 7 (and possibly later versions from what I've been told). They worked on a lot of different Mac models. Unfortunately, these are pretty much obsolete now, as newer iMacs and MacBooks have microphones built into their bodies.
But this thing does have a use in PCs! If only it were standard though. Look at that weird plug. See it? This is what's wrong:
Internally, the mic had a sweet amplifier that was powered with five volts DC by the mic jack itself. Unfortunately, it wont work if you try plugging it into a standard machine. This is because of the pinout on this plug. It's still looking for 5 volts at the tip. So you've got to rewire this thing to a 5 volt source to get that amplification working again.
PlainTalk (proprietary connector):
GND | Mono sound |+5v\
Regular Mic (1/8" or 3.5mm jack):
Ground | Rght | Left \
So you can see that you need five volts. But where do you get it from?
You know what else in a PC is 5v? USB! So what we're going to do is cut the USB 'b' side of a cable off and wire the 'a' side for power to the microphone. Once we do that, we'll attach a normal 3.5mm (1/8") jack to the microphone. Jamming it back into the original case is certainly something you can do after we're done if you're into the whole retro thing. Just peel up the padded bottom of the microphone, and there will be a single screw holding it together. The plastic case pretty much falls apart after you take that out.
Now we're cooking with USB! (Ignore the silver wire to the upper right—the board was slightly damaged when I was prying off the metal shield, so I needed to connect a ground wire. You don't have to do that, but do connect the other grounds.) You can cut and tape off the USB cable's data lines (Green and White). We only need power, after all.
Finishing all of your soldering, you should be able to plug the microphone into a computer. You can try it out immediately, but I suggest doing it on a junk computer or stereo with a mic jack, just to make sure that your wiring is correct and that you wont fry anything with an accidental short. Once you verify that it's working, you're goot to go! You can drill the side of the original plaintalk shell out and put it all back in there (with a small hole for the new USB power line), but I wanted to go more obscure. So with a few zip ties and some old, familiar friends, I constructed the ultimate in desktop accessories: