What happens to the transponder, FDR and CVR in case of an explosion?
As ratchetfreak noted, the recorders don't transmit data, attached to them is a location beacon that operates for up to 30 days.
This location beacon isn't always successful in enabling searchers to find the recorders. In the case of AF447, it took two years before a search located the aircraft debris field on the sea floor and recovered it's recorders.
As for withstanding the stresses of an explosion, it obviously depends on the strength of the explosion and it's location in the aircraft. In the case of Pan Am 103, destroyed in flight by explosives, the recorders were recovered and data extracted.

A DFDR must obviously have a high degree of ‘crashworthiness.’ The first units were required to withstand a momentary shock force of 1,000gs, the latest test standards now call for a test to 3,400gs for a duration of 6.5 milliseconds. The units also have to withstand a static crushing force at all of its six axis points of an applied load force of 5,000lbs for 5 minutes on each axis. Third, it must withstand a 500lbs piercing force test conducted by dropping it onto a ¼ inch steel pin from 10 feet. Lastly it must withstand a 1,100°C fire test for 60 minutes, and a 260°C oven test for 10 hours.
It is also required that these units are mounted within the tail area of an aircraft, away from the potential crushing force of any engines mounted nearby. The DFDR must be watertight to a depth of 20,000 feet in sea water, and survive at this depth for 30 days - and it must be fitted with an underwater locator beacon which will act like a sonar transmitter, by ‘pinging’ a signal through the medium of water that it might be laying in.

From http://www.avbuyer.com/articles/detail.asp?id=1858

1G is the force you feel on your feet standing up.
12G is the most a fighter-jet pilot normally experiences in combat.
16G sustained for 1 minute will kill you.
25G for an less than a second will hospitalize a fit racing car driver


350G results in total body fragmentation




In a 727 crash landing

Impact forces at the front of the plane peaked at 12 G (12 times the force of gravity), in the centre the force was less, 8 G, at the rear it was 6 G, half that at the front and no greater than being hit in a fairground bumper car.



In a 720 crash,
the maximum force was 14 G vertically for 0.8 seconds


This F4 phantom striking a concrete block at 480 MPH produced average peak forces of less than 60 G (see fig 7). The most solid elements (the engines) experienced forces of up to 700 G (fig 11).


3400G is therefore a lot.