I have used these before for small projects like this. If you can get this or a similar audio amplifier module it should work pretty well. I like them because the connections are all clearly labeled and saves me the trouble of building a circuit from scratch. I haven’t figured out how to add a volume control yet, but I’m sure it’s pretty straightforward.

Good luck!

Edit: Actually, looking into it more, this cheap amplifier probably won’t work very well for this application. You might need a pre-amplifier circuit for the microphone first. I also don’t know anything about modular synths. Take all my advice with a grain of salt. Make sure to do lots of research.