@kezo, I have three cables, all marked High Speed with Internet. Tried them all between the TV and AV receiver, no difference. Also unplugged the other HDMI devices, leaving just the one cable. Also no difference. The Panasonic passed the volume change without any delay, so I think this is a software issue in the Hisense.
Since checking for firmware/software updates when you first had your TV, have you checked recently?
When you first got the TV, I remember you wanted to test the internal speakers. With the AV reciever out of the loop did CEC behave as should with your firestick and blue-ray?
Concentrating soley on CEC for a minute – On your AV is it possible to to turn ARC off and still have CEC ability. If can’t do this swap HDMI ports with your firestick on the TV (firestick HDMI 3 – AV HDMI 2) You should still have CEC control on your AV.
Can you put pictures up of what settngs are available in ARC settings (on/off) if this is available – Audio setings and Advanced audio settings pages on your TV please.
I’m going to have a food read of your AVR manual Ive downloaded later wheb mrs kezo is watching the Saturday evening. rubbish.
Don’t give up you aren’t the only one with problems and certainly won’t be the last. As the saying goes theres more than one way to skin a cat (no pun intended) 🙂