Hmmm, don't know if you want to go there!!
I downloaded some nice AI.acft sounds long ago and really made the flyby sounds much better.
But, the sound files you are talking about are in the CFS2 Sound folder.
Cl1_ext, Cl3_ext1 and Cl3_ext2. They are all .wav files. That's C L.
The Cl4_ext might also be involved.
To test this, just rename those files to say........Cl_ext.orig etc. for those files. Or even cut and paste them onto your desktop. Try the game and see what you get.
Don't do anything until you protect those files.
If you already lost em, then they are located on the CFS2 disc, BUT YOU have to look for them.