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 original sound files have been preserved by renaming to
*-stereo.wav, as the information content in both stereo channels are similar but
non-identical, and mono versions created by extracting the left channel:
- cabinalert.wav
- flaps.wav

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:
- touchdown.wav