Looking at your schematics, my guess is that you are only providing a very light loading on the switching psu which may cause it to have problems regulating and also might present some switching noise to the supply rails. To further compound the problem, you have nothing in the way of a reservoir capacitor on the regulator, 0.33u is way shy and will struggle. Try placing >100uF before and after your regulator, say 470u before, 220uF after. If the mains buzzer is putting out a lot of noise you may also have to place a sensibly rated 100nF capacitor and 1/2W 100R resistor in series across it and an inline filter in your 5V rail to clean up any noise on the supply rails. Any negative going instantaneous spikes on the supply rails might well cause the PIC to either latch up or reset.
If the buzzer is really noisy electrically, you might also need to place a TVS across your 5V rail to cut the peaks off the spikes first and then allow your filter to clean up the rest.
rgds