Looks like there's an issue where the audio is actually just mono, but is split into stereo channels, but after that point in the video, one of the channels is

Looks like there's an issue where the audio is actually just mono, but is split into stereo channels, but after that point in the video, one of the channels is inverted. So if you're listening to them through a single speaker or otherwise mixed down to mono, the channels will cancel out. Try using headphones or stereo speakers, or download the video and play it back in a program like VLC that lets you select just one of the audio channels to play.