You are correct, if it is only on some channels it cannot be the connections, and if you google the topic you will find that this is a known issue with some but not all directv channels that is most likely going to take a while to resolve.
For example Channel A & E HD on Directv has a beautiful picture but the lips don't sync with the dialogue (almost a 1 sec delay?), I read that the problem is sometimes caused when excellent video images that take a while to process are transmitted but the engineers do not figure in an appropriate delay for the audio signal.
The only way to fix it is to add some delays channel by channel for the audio if there is a program you absolutely must see, or otherwise ignore it/skip the program until the directv engineers get their act together.
