How to make active buzzer louder
Put the buzzer in the collector circuit and it should be about as loud as it is connected directly to the 9V source. If that's not loud enough, get another buzzer. Hopefully it's rated to work with 9V. 







simulate this circuit &ndash; Schematic created using CircuitLab



You were connecting it as an emitter-follower which gives you about 0.7V less than the logic input voltage (4.3V with 5V logic) and the remainder (4.7V with a 9V supply) wasted in the transistor. A higher supply would just result in more wasted power. Depending on how much load the buzzer represents the emitter follower may result in a bit less loud than a direct connection or a bit louder (since the MCU output will be loaded significantly by most buzzers and you won't get the full 5V out).