When you first turn the machine on, you should hear sound, regardless of whether the MPU has booted. The audio board boots on its own first. Do you get that sound, or just no audio during game play? If you get a boot sound, then it is your interconnect between the audio board and MPU, they're not communicating.
If you get nothing at all, try wiggling the volume pot back and forth first. Can you hear any static or hiss coming from the speaker when the volume is maxed out? If so, the audio amp itself is probably okay. A poor pin connector to the speaker is a possibility, as is a blown out speaker, but in most cases you will still hear something like a distorted fart unless the voice coil is completely melted down. If you have a 4 or 8 ohm speaker laying around like a car stereo speaker, you can remove the wires to the cabinet speaker and connect it to that to check and see if that is indeed the case. At that point, it is worth installing connectors to the speaker wires so you don't have to solder directly to the speaker tabs. No static or hiss at all even with a different speaker and the connectors solid, then it could be a faulty TDA2002/2003 amp or a failed cap(s).
If that all checks out, then it could be anything from bad EPROMs to faulty CPU, RAM, Clock/xtal, D/A converter. You name it. There are ways to check all of them. How do the sockets look? Anything rusted up or corroded looking?