This video shows how to replace a factory-installed ESU Essential Sound Unit Decoder with an ESU Loksound 5 21-pin DCC decoder in a Walthers Mainline GP9.
Please note, the decoder purchased was a blank decoder and you would need to load your own sound file. The Loksound dealer I purchased this from was kind enough to install the GP9 sounds on the decoder for me.
Disclaimer: This video is for entertainment purposes only. The author is not responsible for any damage/issues by anyone trying to replicate the demonstration. Please consult the instruction manual(s) before attempting a decoder swap.