Files with specific file names could be used to replace the standard sounds in the car. For example, the chime for unbuckled seat belts could be replaced by a voice message saying whatever you want. To set it up, place files with specific file names in the root folder (or a specific folder name) on the USB stick.
The file name could identify the sound like: TeslaDoorOpenChime.mp3 or TeslaPlayOnExit.mp3.
This could open for all kinds of custom audio alerts, such as sound played on exit, turn signals, etc.
Since this could possibly be used to disable the seat belt sound, there might have to be some analysis of the sound to prevent this, as I suspect there are laws requiring this sound to work. For this particular scenario it could play the regular chime, then the audio for maximum X seconds, then the regular chime twice, then custom sound and so on.
Should probably not require anything but software changes, but may perhaps conflict with laws in some regions. Particularly regarding the seat belt sound.