your buzzer turns-on for only 1 second, but I can tell you why you should use a PNP:

Please have a look into the datasheet of your micro and look what happens if the micro is reseted. You will find, that (when using a standard 80C51) all port lines are set high (P1,2 and 3), respectively set off (P0). That means, that your buzzer turns-on when you reset the micro, for instance, during power-on. So, whenever you power-on the micro, the buzzer will be activated for a brief period of time. And if you use a reset chip, then also during/after power dips and power-downs your buzzer will turn-on for the duration of power failure occurence (enough supply voltage assumed, of course) and reset active time. Do you want this?

Another reason to avoid the NPN circuit you have chosen is, that the base emitter turn-on voltage will make tear down the potential at port dangerously close to the trip point, where the internal port circuitry of 80C52 turns-off the weak internal active pull-up, to save current. So, what works with your current set-up might fail with your next.

This issue was already discussed:

http://www.8052.com/forum/read.phtml?id=120493

Kai