hi,
we're doing this in physics now, so i'm trying to explain you:
I've scanned an image for you to make it easier to explain.
the crystal is between the 2 metal plates. When you start talking, the plates were pressed to each other, and because of that, you can measure a voltage between them (the crystal is 'flatened'). As you can see, on the right side of the picture, the upper side is '+' because there are two crystal parts '-' (+/- goes to each other, while +/+ (and -/-) pushes off.) of course, a crystal mic doesn't really work with one single crystal, but theoretically it could, but you can't amplify one e-.
a crystal speaker works paralell, with the difference that you put voltage on it.
i know its very bad formulated, but i hope it could help you, and i'm sure in a few weeks i know more, perhaps you write me and i'm trying to help you on.
if this wasn't enough, search for 'piezoeffect' @ google.
cya, sankt-seibel