Alright after 2 years without a capable PC im back. Im currently investigating/modding NFS Pro Street .tmx sounds.
The tmx has both gin files and the snr format which can only be opened in audacity but some guy on this forum said it wasn't possible to encode custom files back, need confirmation on this one though. The game has no issues reading modified tmx files, gin files can be any size or length.
SNR format is pretty much used for tones of the rpm range, much like the .abk format in previous games so unless someone makes an encoder, we're not 100% able to accurately modify sounds.
There are some float values present in the header, some very obvious ones which im 90% sure they are related to the rpm range of the sound set and others which have unknown purpose but I still think they're related to sound properties only. Luckily there is no assembly code in the .tmx format unlike the older .abk.
On the subject of using custom gin files, it isn't as straight forward as in NFSMW or Carbon, where you could hear the modified audio clearly almost with no changes or differences. In this game it's different, the audio files change a lot. (Needless to say that there are much more in-game parameters to try and modify from the database using this tool https://nfs-tools.blogspot.com/2017/09/nfs-vlted-v45-released.html I didn't even start playing with it though)
But going back to the files, it's hard for me to really pinpoint what's really happening with the audio. My test is simple, I swapped the gin acceleration file with a locomotive idling sound.
It's easy to recognize the sound in game, but it's morphed to something entirely different; I will upload a short video tomorrow maybe.
If someone wants to start testing things up just reply here, im gonna see what's possible with this game