Knowing we need to amplify the signals for the speaker, we built a small Darlington amp using transistors. It wasn't terribly great. The voltage loss was high and we didn't get a usable amount of audio. We are not yet happy with the final results of the speaker volume so are now researching a flat piezo speaker to see if that will produce better results. From there, we plan to then miniaturize the speaker, which will require a change in the driver circuit.