For looking at audio, a cro.
Do you have audio at all? Have you tried on a HDMI input or composite input? Have you tried the audio outputs as well as the speakers?
If you have no audio at all, I'd start by hooking up a sig gen (1khz sine wave) to the L and R audio inputs and just chase down the signal with a cro. You should be able to find it at the mux and then follow it through to the processor and from there back to the amps. When you find where it disappears then you've got a good place to narrow down your search.
It goes something like this:
The audio inputs go to IC771 (TC4052BF), which selects between them (outputs on pin 3 and 13, L+R respectively) and sends them off through CN302 (pins 13 and 11) to the main processor board.
Chances are that this part of the circuit is all working fine though, cause this only handles audio inputted to the TV, not audio that's been received through RF, so assuming you've no audio at all this should all work fine.
From there the audio goes from the main processor back to the power supply PCB through CN301.
For the audio outputs on the back of the TV, the audio goes through pins 2 and 4 (again, L+R respectively), through IC871 (op amp with a small gain) and to the output jacks.
For the speakers, audio goes through CN301 pins 6 and 8, and goes to IC801 (audio amp BD5426EFS) pins 1 and 44. Power supply for the amp is 13V on pins 29 and 30 (doubled up, they're internally connected).
See how you go with that and let us know your results.