Hi @mcstringer ,
It may be a that it is a faulty audio amp circuit in the TV although this doesn’t really explain satisfactorily why there is intermittent audio only in the headphones and not the main speakers as well.
Here are some poor images (apologies) taken from the service manual that show that the audio is fed out of the TV’s CPU to the audio amp and then to the speakers and that the headphones have their own amp.
(click on images to enlarge for better viewing)
Sorry to be so vague but the TV will have to be opened and the problem further diagnosed as to the cause.
Note: The service manual can be used online. It is awkward however as you have to select the page number at the bottom below the document box every time that you wish to turn the page. To download it you need to register with the website.