Ok, hers how to do this:
1. Alias your plane to a default FSX aircraft. I'll give you some generic ones and you can copy and paste this into your sound.cfg folder. but how to get there? yourdrive/program files/microsoft games/microsoft flight simulator x/ simobjects/airplanes/yourplane/sound. Delete all files in the sound folder except the sound.cfg file. Then open up sound.cfg with notepad. Delete everything in there and copy and paste either of these choices in. (you only need to put in 1)
for large airliners
[fltsim]
alias=B737_800\sound
for private jets
[fltsim]
alias=Lear45\sound
for medium prop planes
[FLTSIM]
product_code=FSIM
alias=beech_king_air_350\sound
for small prop planes
[FLTSIM]
product_code=FSIM
alias=c172\sound
Once you've done that, go try your plane out. And Wa-la, new sounds for your plane!
****AND MAKE SURE YOU READ ALL OF THESE STEPS CAREFULLY****
because it's no fun messing up your FSX and re-installing it.
Hope this works for you

...Oh and lol sorry about the long post.