I had the same problem discussed in the forum - got tv picture but no sound on amp.
I was thinking all the possibilities and even wanted to find a headphones to plug in and out.
After I saw the post above, I came and hit a little the headphone jack and the sound turned on!
My receiver 1910. We have very hot summer and I assume smth switched in the jack because of temperature (but this is just a guess). Stil it helped!