@Bsmooth If you want to modify something in an existing Mod, i would recommend to not to edit the Files in the original *.MAS Files.
Extract the Files that you want to edit, put them into a new *.MAS File and create an Update rfcmp for the Component.
In that Way, you just have to move the Subfolder with the Update from the Installed... Folder, to avoid Mismatches in Multiplayer.
When you want to play Single Player, simply move the Folder back.
rFactor is using *.wav Files too, so that is no Problem, they just should be mono and not stereo.
But in rFactor the Sound File "must" use the Extension *.SFX (old) oder *.JSON (new Sound Scheme), so you have to search for these Files.
If the Sound Files are *.ini Files, check the *.veh Files, if they are linked as *.ini there.
The Line in the *.veh File is: Sounds=
If they are GTR2 *.ini Files, they should have the same Format as the rFactor *.SFX Files, so you can edit them in the same Way.