I have tried the following with MY particular "picture with no sound" issue (since Samsung HD Tech seems to think this problem is case by case) and it has worked:
1. Turn on the T.V.
2. let it run for approx 30 minutes (use your mute button if you don't want to hear the sound)
3. Turn off the T.V. and let it sit for approx 10-15 minutes
4. Turn it on.
With my LNT-4661 this has worked for me everytime. I could be damaging the TV but it's better than me damaging it with my foot out of frustration.
I hope this helps anyone. It's at least worth a try.