Lets try the simplest solution, first. Go into Windows Control Panel, select "Sounds and Audio Devices", and click the advanced tab under "Speaker Settings". Once there, make sure that you're using the correct speaker setup. That is, if you have 2 speakers (or a sub with 2 satellites), choose "2 Speakers", or If you have 4 or 4+sub, choose "4" (etc.).
If that doesn't help, Extract the flightengine.ini from the Flightdata.cat, open it up in notepad, and scroll down until you find "SoundSettings". Edit that section to match this example:
[soundSettings]
SampleFrequency=44100
BitsPerSample=32
DopplerFactor=1.0
While it admittedly seems a bit extreme, the latter suggestion has fixed the issue on quite a few systems with high-end sound cards.