Youtube sound suddenly disappear

I had the same problem of no youtube sound with one firmware before the last update. Apparently it had something to do with audio settings. I have to turn off “AAC compatible receiver” to make youtube sound reappearing. I had no problem ever since.

Then comes this last upgrade, I upgraded. Tried the new youtube with new interface. No problem with sounds at all.
A few days later, suddenly this no sound problem appeared again. I checked audio settings, aac compatible receiver is turned off. So no changes.

Can anyone help me shed a light what is happening ?

Before anyone answering, I would like to inform, I am using optical cables for audio connection to my home theatre system.
Changing to stereo would send the sound to tv.