Dear friend,
This may help you:
1) Try using negative logic while driving the buzzer by connecting the positive terminal of the buzzer to +3.3V and negative terminal to the GPIO of the Wemos board. You will have to output a "low" to activate the buzzer .
2) There won't be any problem with using the buck converter, but make sure that the converter is a good quality one with isolation. This will ensure that high voltage side and low voltage side are properly isolated. No issues with the current rating as higher the current rating , better is your power supply.
( I assume you are using an active buzzer)