There should be no sound when all the voice and sine wave rockers are turned off.
It will be worth checking all the contacts on all the Orbit rockers, to see if any are making when they shouldn't be. The Orbit, IIRC, is basically built around one IC. Even back in 1973, Wurlitzer UK were not servicing the Orbit board but just replacing it, with the faulty item going back to Wurlitzer for attempted repair. If that IC is faulty, then that's probably the end of your Orbit. Let's hope it's something more simple.
Service manuals may be available from Organ Service Corp. or W D Greenhill.