FlightGear - Flight Simulator FGAddon

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 sound files have been converted directly to mono, by extracting
the left channel and overwriting the original file, as both stereo channels
contain identical information:
- Sounds/flaps.wav

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/gear-up.wav
- Sounds/running.wav