I'm not familiar with Tom Gibson's models, but if you have a question about the sound configuration of an aircraft, I recommend looking in the aircraft's Sound folder for the file Sound.cfg.  You can open it in Notepad. 

If the file gives an alias configuration for the sound file of another aircraft, then you will hear the sounds of that other aircraft - assuming you have it installed.  This might be what's going on with the aircraft you're working with in regard to the Cessna sounds.

If the file contains settings for different types of aircraft operation, then it would require all of the proper .wav (audio) files to be in your fs2002/Sound directory for the sounds to be produced.  If your add-on aircraft came with a sound package, make sure all of the included .wav files are copied in to your fs2002/Sound directory.

Good luck!