I haven't had to do a lot of debugging on the WPC MPU board so take this for what it's worth...
If the sound is working then that suggests address/data are getting from the MPU to the A/V board. (Although it's possible there's a higher order address bit that could be stuck and preventing data from getting to the video address space.)
The more likely problems are an issue with the FIRQ line that comes back from the A/V board and the BLANK that goes out to the AV board.
FIRQ is pin 34 on J202
BLANK is pin 27 on J202
If the FIRQ isn't being detected properly on the MPU, it will interfere with the display pages being updated. You may want to check to make sure that signal is toggling on the MPU. You will probably need a logic probe, or oscilloscope to confirm it's toggling.
The other really important signal is the BLANK since this is logically NOR'd with another signal on the A/V board to produce the DISPLAY_ENABLE that goes out to the DMD. If the BLANK is stuck high, the display will never light up.
Both of those issues could be caused by a defective U9 or poor socket connection. You can try swapping U9 from the working board to see if it resolves anything.