In a normal situation, the pilot flying (PF) controls the aircraft and the pilot monitoring/pilot not flying (PNF) handles the radios.
In a situation requiring use of the QRH, the PF takes control of the radio while the PNF concentrates on the checklist/2nd radio back to company/etc.
In an urgent situation (decompression, double engine failure (single engine failure is not considered urgent), extreme weather conditions on landing) the captain, at his discretion, will take control of the aircraft. This is due to him being the PIC, generally (but not always) having more experience in the aircraft or industry, and being able to make split second decisions about the immediate operation of the aircraft.
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With AWE1549, the F/O was PF. At the time of engine failure, instead of the F/O taking the radios, the Capt took the aircraft. That is why you don't hear any change of voice before/after the emergency. This, in no means, implies the F/O was not capable of handling the emergency on his own. The F/O realizes everything comes down on the Capt's shoulders, and the Capt usually has more experience with the controls on the aircraft. On a few occasions, I gladly turned control of the aircraft over to the capt to allow his skills to get us back on the ground (plus, I was just tired of flying in 60+ knot winds).