I got the alias for the sound wrong in the FS9 B747-400ER download. Download the attached file and put it in the sound folder for the B747-400ER overwriting the file currently located there.
If you have installed FS9 to its default location the 747-400 sound folder is located at:
C:\Program Files\Microsoft Games\Flight Simulator 9\Aircraft\WW Boeing 747-400ER\sound
EHAM-YSSY B772LR