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This problem is most likely caused by a fault on a buffer board, which is in turn causing the Y-SUS Main board to fail.
Both boards will need to be replaced as a set when this happens.
Yes, this can also be responsible for no sound coming from the TV.
To confirm, turn the TV off and unplug it from the wall. Wait 5 minutes for the capacitors to discharge. Then unplug the connector going to the Y-SUS board and power on the TV. If it now powers up with sound, this confirms that the faulty Y-SUS board is causing the sound loss.
If you unplug the connector going to the Y-SUS board and power on the TV, can you hear sound?
Ok, I will open the question up to the other experts to see if they have any idea on this...