This follows from http://thread.gmane.org/gmane.games.flightgear.devel/77932.
With the introduction of the 3D sound engine, all sound files in FlightGear now
must be mono point sources.
The following original sound files have been preserved by renaming to
*-stereo.wav, and mono versions created by mixing both channels (as the
information content in both stereo channels are different):
- Sounds/an2_flaps.wav
