I believe from memory there's a 7905 regulator for the -5V.
The audio stage runs at 24V, from +12V and -12V only. The 7905 regulator is only provided for Zorro expansions that might happen to use -5V, nothing to do with the audio stage.
Looking at your measurements, the problem is that the -12V supply is missing from both op-amps. From memory, the track for this physically runs under and around the cluster of three 47uF capacitors, so you'll probably find the track under there has corroded open circuit somewhere. Will be easy enough to find without the capacitors fitted.
When done, you should measure about 1 Ohm between U402 pin 11 (and U403 pin 4) to the -12V pin on the board power connector, which is the middle pin on the side that's not ground. I can't remember the pin number and I'm too lazy to look at the schematic.
