hello ian
i havent had both these faults at once on the same set,but i have have them on different sets ,
the transformers on the invertors can become thermaly sensitive,warm both boards up, the backlight should stay on then freeze each transformer in turn you should be able to find the faulty one,
as for the audio fault this can be caused be the main signal pcb, check the larger of the ics if they get v.very hot very quickly you will need a new pcb.and dont forget to check the caps in the psu.
hope this helps