Scott says it could be a heat issue. It could also be an issue that can be solved through an external audio source like a sound bar or home theater system. Could it be a problem with the satellite receiver? Scott says that unless the TV before the Vizio had the same problem, then probably not. Check for LipSync control in the menu. Scott recommends taking the TV back since it is still under warranty and let them repair it.
Pete calls in and says it may be the satellite receiver. Play some DVDs or Blu-rays and see if the audio goes out of sync. If it doesn't, then it does point to the satellite receiver as the issue.