I doubt it's a codecs problem if you can hear some FS sounds but not others because they are all the same format.
You can easily test this by navigating to C:\Windows\Media and trying to play some of the .wav files in there. If you can hear them it's not a codec problem and I'd check the program - settings, aircraft folders to make sure the files are there, etc.
If you can't hear any sounds at all I'd suspect a sound card driver problem before I'd suspect a codec problem.
If you really need codecs you can get them here
http://www.free-codecs.com/ No sound at all anywhere = driver.
Some sounds but no .wav sound = codecs.
Some sounds in FS but not all = FS problem.