Search a video on YT with "Gran Turismo 2 AI sound" or something. The guy drive the following cars: GT40 Race Car and RX-7 LM. In the comments there is the code, but is for PAL. On Google search for the "Gran Turismo 2 code converter" and the code is for your versionI am runnig the 2.1version of the game.
I have still for a long time looked into it but it didnt answer
Thanks a lot sir.