This is audio catching up with visual in the world of HD and digital. It could be a number of reasons your TV does that.
First off, if your TV is more than 5yrs old, you may need a new one. (yes, TV's built in the last 10 years were built to fail, anticipating the recording upgrades. I think they are better now.) Because:
Digital is instant. Both audio and visual are recorded that way. Light is faster than sound, regardless. On the channels you do have sound, do the lips ALWAYS match with the words? It might be slight but, I bet they don't. (I notice on some shows at my house on a 7yr old TV)
Also, there are recording laws as to what level of "noise" you can record at. (there are no such laws for commercials, that's why they are so much louder than the show your watching. (on the channels you can't hear the show, you can hear the commercials, right?)) Because digital is "cleaner" from the git go, the levels at which it can be recorded are lower.
Your TV may not recognize the new recording/broadcast formats. Some, it seems but, not all. And the ones it does recognize, might not happen right away. Turning it off and on might help the lagging in recognition but, it will not fix it. In a nut shell, the broadcast may be too quiet on the waves your TV uses to let you hear it.
I hope this helps and didn't just muddy it up for you.
lol