Well there are two reasons why you hear this
buzzing.

1) When leaving the headlights on and removing
the key.
If you then open the door the buzzer will sound.
This is to remind you that the headlights are
still on.


2) When you start the car and you don't have
your seatbelt on, the buzzer will sound.


Knowing this you can track down the problem.
If the buzzer works in situation 1 then the
buzzer is OK and then the problem is in the seatbelt receptical(s).
If not, check the fuse.

Louis.