Never measured it, probably wont find anyone that has. Only way to find out for sure is to open it up and take a measurement. Doesnt take much effort to get that far.
If I was forced to make an informed guess I would say 5 volts, anything more is a stupid design as that is plenty and already available on the board. Only other would be 12 volts but that's not likely or necessary. If you really don't want to take it apart twice then you can order up parts based for each and send back what you dont need when you are done of keep them for something else. Small Led light strips are dirt cheap.