I suggest that you read the gmax to Fs thread, but obviously you've got the basics.

aircraft.cfg
In your airplane's folder you'll find the aircraft.cfg file.  This is one of two key files (the other being the *.air file) that define the airplane's flight characteristics, with teh aircraft.cfg values being more easily modified (with a text editor).

You can have the visual model you want, but it's the information in the aircraft.cfg and *.air files that define how it flies.  (Yes, you can have a 747 fly like a Cessna)

The Aircraft Container SDK specifies what goes into the aircraft.cfg file.


model.cfg
This file is in the MODEL folder.  It's usually a very simple text file stating that the "normal"  model uses a specific *.mdl file.  As you've found out, the *.mdl file is the visual model created by exporting the gmax file.

sound.cfg
IT can be a simple text file, if your model's sounds are aliased to a default - or other - aircraft.  If you have sounds specific to your model, then you define the sounds in this file.

panel.cfg
This can be a hairy file.  Unless you alias it to another aircraft (usually one of the defaults), this is where you specify what instrument goes where, what bitmaps are used for your standard (2D) and stacked panel windows, etc.

Check the panel SDK for information.