Could you be a little more specific as to your details. Do you know how to open a sound.cfg file with notepad? A lot of downloaded planes sound files are aliased to a Microsoft plane. This file will tell you if it is or isn't. Very few come with their own sound file.
  Post again, if any more questions arise.

Using notepad, which you can use to open any .cfg file in the FS directory.  However, if you do not know anything about the contents of the file, I wouldn't mess with what's in there.  Go ahead and open it, but don't change ANYTHING until you get a comprehensive lesson on the subject.  I myself know diddly about the sound.cfg files.

I do know that if, when you open the file, it says something like "alias=learjet45" then the sound file is aliased to the default Learjet.  This is a very common aircraft to alias a sound set to, so I would imagine this is somewhat the case.

If the sounds are aliased, go to your aircraft.cfg file for the aircraft that is specified and open it with notepad.  Look for an entry very near the top that reads "title=learjet45" or whatever the name of the aircraft is.  If that line and the "alias=someaircraft" line in the sound.cfg file don't match, then there is the problem.  Just copy the title over and that should take care of it.  If it doesn't, try aliasing to another aircraft with the right sounds, using the same method.  ALWAYS REMEMBER TO MAKE BACKUPS BEFORE CHANGING ANYTHING IN THE .CFG FILES!!!!!  YOU HAVE BEEN WARNED!!!!

Now, as I said before, I don't really know diddly about the sound file, I just put this together from what I've learned around here.  Meaning, I could be wrong, so don't blame me if something goes awry . . .

Exactly.  Just insert the title of the aircraft you wish the sounds to be aliased to.

In the aircraft.cfg file:
[fltsim.0]
title=learjet45

Insert into the sound.cfg file:
[fltsim]
alias=learjet45\sound

LOL!  Like I said, I don't know diddly . . .