It seems to be some technical fault with your TV and you must take it to the repairer as they will open it and will check it, because according to your question how does it possible that your TV go green and then shut off but you can hear sound after that, because after shut down the TV is actually off then how can you hear sound? So there must be a problem do take your television to the repairer. And if it happens that your TV go green and while that situation you are hearing sound and after that your TV got shut down then there could be a problem with the picture display/colors or a sleeping mode.
It seems as if you are having a problem with your picture tube. Picture tube is the main part which is responsible for showing images and colors on the screen. If you can't able to see any image then there is a problem with the picture tube. But it is better to ask the supplier to examine and fix it for you.