Ok, spent a few minutes with the multimeter, I also followed a few instructions from pinwiki. FYI I had already done the ground modification in the cabinet (remove molex connectors, ground the wires to the metal frame).
So, checking the power supply board was fine, followed pinwiki instructions:
"When measuring the +5VDC provided by the power supply, first disconnect J2 on the power supply. Measure power at the J2 male headers first. Once within an acceptable range, turn power off and reconnect J2. Measure power across C1, the 100uf/10V capacitor that is next to J1 (power connector) on the MPU. The MPU may drag the +5VDC supply down a bit. If you find this to be the case, adjust the trim pot on the power supply until a steady +5VDC is seen at the MPU. Poor connections between the +5VDC power supply and the MPU board may also reduce the voltage measured at the MPU. "
So things I checked:
1) Voltage at J2 was ~5VDC, voltage at capacitor C1 was ~12VDC.
2) Then checked Reset pin #9 on A6P1 (connector on sound board), was ~5VDC (pushing the SW2 button had no effect on voltage at the reset line).
3) Unplugging display connector had no effect on sound boot.
4) Noted sound board dip switches were On-Off-Off-On (1-2-3-4), changed to Off-Off-Off-On since how they were depicted in the schematic (open open open closed). Not sure if I did this right, just seemed to me they should match the wiring diagram.
5) Noted that the A6P1 (connector on sound board) pin 12 and 17 are bridged, this is bridging the AC and DC ground as per wiring diagram, I assume this is okay, but not 100% sure there's not a ground loop or noise being introduced.