You haven't said what you have tried yet so first make absolutely sure it's not user error: voice levels are turned up, attack turned down, instrument switches pushed in, etc. However, the Omni is notorious for reliability issues. If no experience troubleshooting electronics, the Omni isn't a good place to start. But if you have the courage and some electronics experience it seems to me the keying circuits and output VCAs are doing their job. The voicing cards may be the culprit not producing a signal. Or perhaps the effects section. Definitely check the power supply to see it is outputting the right voltages. I've had old synths with connector contacts becoming oxidized and requiring cleaning. I always say there's no shame in taking the synth to a reputable tech.