If the speaker amp is sensitive enough, you should be able to use it. This is just off of the top of my head, so probably not entirely correct, but it should work if you supply power to the microphone over a resistor (the resistor should be a few kohms, the negative lead of the microphone (the one where the traces go to the case) goes to negative and the other lead over the resistor to positive.) You'll then need a small capacitor (not really sure about the size, 100nF should probably work somewhat) going from the positive lead of the mic (so between the mic and resistor) to the input of the amp. The resistor limits the current through the mic and the capacitor only couples the AC, so the DC offset gets removed. If the speaker amp can pick that signal up, it should work. Otherwise, you could use an lm386 to amplify the microphone, it should also be able to drive at least a small speaker. You can also try just connecting the microphone between the input and ground of the amp, that could work too.