In a word, don't. It isn't worth the install in the GP9
The Athearn Blue Box line of locos suffer from the live frame disease. To make it operate at the best of it's ability you would have to;
1. Bypass the electrical-mechanical connection between the trucks and the frame.
2. Swap out the stamped-steel wheels with Nickle Silver ones.
3. Isolate and possibly replace the motor. They are Amperage hogs and can easily overload and destroy your 100 dollar sound decoder.
4. Toothpaste the gears or you wont be able to hear the sound of the decoder.
5. Install a larger capacitor to try and bypass the limitations of the Athearn design.
All of the above take time and the end result is still a loco that has a body that is too wide and still wont run as well as an Atlas or newer Genesis GP. A sound decoder is an investment. Installing one into a unit that wont give you great results is a waste of time. When you run the loco, you will see it run and stall and run and stall. That typically is ok with a standard DCC decoder, but with sound you will also HEAR it.
That is my advice from a professional installer.
In terms of a Digitrax upgrade, consider the Zephyr system. It will have more than you need in terms of operating capabilities.