According to “When the Factory Reset of the unit is requested?” the files should be preserved, if I understand correctly. But I would say be prepared to lose the data anyway.
Edit: A factory reset deletes files in “/data/INTERNAL”, too, as @Buzzzz reported.
If you want to avoid any risk that use another SD card to install a new Volumio system for testing purposes. I would say that this would be the preferred way anyway.