1. Why are cockpit voices recorded only in the black box? If other data from a plane is constantly being sent for maintenance purposes, while in flight, why do planes not also not relay cockpit voices to some storage system, for just such a situation? CCTV keeps footage for a few hours or a few days, why is this kind of valuable data not also routinely stored?
2. I don't know how low you have to fly a plane to fly "under the radar" but isn't is possible that the pilot or a hijacker flew the plane below radar to somewhere where it is now on the ground?
3. One other possibility: the plane was shot down, and the country that did it, realizing the mistake, is participating in the search to cover up. Militaries have shot down civilian jets before. The Soviet Union and the USA have both done so.