The first step is to detect the flash. There are two good ways to do this. The classic way is to use a cadmium sulfide (CDS) cell. This is essentially a resistor who's resistance is very high in the dark and fairly low in the light. This sites gives a good example of how to use it with a comparator.
http://www.bristolwatch.com/ele/pd.html
The other way to detect the flash is to use a diode. If you apply power to an LED it lights up, but if you apply light to an LED it will actually create a small voltage difference over its two terminals. This voltage can actually be fairly significant, but the LED cannot support any appreciable current. This is why you don't see any LED solar panels. There is also something called a photo-diode that works the same way, but more effectively (these are used in solar panels). The circuit on this site uses a photo-diode and a comparator with the LED as the output indication. If you want to try it with a normal LED as the detector I would omit the 2.2M resistor.
Both of these sites should have given you a good idea of what the comparator is for, but here is one more. http://www.physics.unlv.edu/~bill/PHYS483/comparators.pdf
Lastly the buzzer is really simple. Just make sure you get one that buzzes as soon as voltage is applied and does not need to be driven like a speaker. Examples:
http://tinyurl.com/9tgmops Amazon
RadioShack.com Official Site - America's Technology Store