I just recently bought the 4th season of Bones and while watching the The Passenger in the Oven I realized as Booth began talking over the plane's intercom that the sound was softer than usual, like it was fading. Did this happen to anyone else?

This happened a few more times throughout the episode and even happened into the next episode. I'm going to try to play it on another player, but it was only those two episodes... Is it just my set, or did anyone else have this issue?(I bought the non blu-ray one that includes all 26 of the episodes on 7 disks)