A different problem, but the solution from this thread FiiO E10K Olympus 2 and windows 10 might work by unplugging it and deleting the device in device mangaer. Windows should re-discover the DX7 when you plug it back in and hopefully goes back to default settings and it starts working again…
Warning : Only go into the device manager if you feel sure of what you are doing though, as deleting the wrong devices could lead to more trouble than “no sound” …