Hi Joe,
it really sounds weird, but that message shows there is a probelm with your folder structure. I mean, our B747 have a MDL, and AIR files named: Boeing747-400-MR-MKIII and not that you wrote. I discovered when there are the same model/air file names among different aircraft folders, FS used to lost simetimes, and generates similar messages, or simply loads an aircraft that he wants... You should check your installed B747s and look for the MDL/AIR file names, also check the aircraft.cfg files for duplicated "title=..." sections among different aircrafts.
For the sound problem, check if you have this folder, that is shown in the sound.cfg: "b747_400\sound" If not, then edit this file and change it to a valid location, where your favourite B747 sounds are installed. In FS2002, this is a default folder so there should not be any problem.