You can download the service manual from here:
http://manuals.fdiskc.com/flat/Rolan...e%20Manual.pdf
There is a way to get it to "display" which voice is playing but the pdf is taking a while to download so I won't be able to tell right away... heading out. (EDIT: Press the PIANO 2 and 3 tone buttons while turning the JX-8P on to enter test mode. The voice being played is indicated by the Solo, Unison, Poly, Volume, Brilliance, and Vibrato LEDs corresponding to voices A, B, C, D, E, F respectively.)
As for the membrane buttons, they are actually tactile switches beneath the membrane. You would have to take the panel apart and replace the failing buttons.