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Did DG audio describe what they did? Fixed a cable/connector? Replaced muting relay? Resoldered a cracked solder joint on the board? It is strange that MIX and Headphone both fail.
Note that with nothing plugged into MIX right, Mix left is the summed L+R mono signal.
Do the A/B/C/D L/R outs still work? These 4 pairs are summed together (analog) and fed to the Mix L/R output and headphone L/R output. First they route to volume potentiometer and return to the Mix/Headphone op-amps. If A/B/C/D work, there is not much circuitry between them and the Mix/HP outs - just op-amps, resistors, mute relays and the jacks themselves.
The only common thing to kill audio would be the muting circuit - that could be temporarily "hot-wired" to unmute and confirm if that is the problem.
By process of elimination, you may be able to isolate the fault and have a local repair shop in Hawaii do the work. Components are not Kurzweil specific. I have a zip file of the K2600 service manual with schematics if interested - that is all a repair shop would need. The board is just op-amps, resistors and the output jacks.
Here is the troubleshooting audio section from the service manual:
Intermittent or No Audio
1. Check the flat ribbon cable at location J808 on the CPU Board to J601 on the Audio Board.
Disconnect and reseat the cable.
2. Check solder connections.
3. On the DSP Board, make sure that U1, U7, and U34 are labeled 7z04.
No Audio, Distortion, or Noisy Outputs
Right Channel
1. On the Audio Board, check the DACs (AD1865R) at locations U2 and U3 input pins 2 and
3 and output pins 12 and 13.
Left Channel
1. On the Audio Board, check the DACs (AD1865R) at locations U12 and U13 input pins 2
and 3 and output pins 12 and 13.
Full volume static distortion after 10-15 minutes of warm-up then locks-up:
1. On the DSP Board, check U2, U3, U5 and U6 (Hobbes ICs). May be heat sensitive.
Different clicks and pops on each effect or consistent ‘thumping’ noise in all
outputs:
1. Clean contacts and reflow solder on Audio Board to Engine Board cable connectors.
2. On the DSP Board, check the signal lines for the components at the following locations:
• Check U25 through U28 (effects RAM).
• Check U8, U10, U11, and U13 (LISA ICs).
Boots up normally but has no sound,
1. On the Engine Board, check the clock signals to the components at the following locations:
• Janis ICs, U17 and U18
• ICs U21 through U24, and U28.
• Sound ROMs, U32 and U33.
Best of luck,
Mike