Vocal Track Missing from Video on iPhone, but not Computer

I've come across a curious problem I can't identify.

For the following video "Ancient Global Warming & What It Might Mean for the Future" (http://itunes.apple.com/us/itunes-u/ancient-global-warming-what/id419193071?i=99 058855) I've discovered that the audio tracks comes through fine on my computer through an iTunes download, but the vocal track does not play on the iPhone. Instead, when playing this video on the iPhone through iTunes U, you only hear the music track. I"ve tested two iPhones (but no other mobile devices) and it was the same issue. The music track is the only track audible through the iPhone speakers, but you can hear the vocal track when you plug it headphones.

Any ideas?

Thank you,

Katie Velazco

Smithsonian Insitution, National Museum of Natural History

http://itunes.apple.com/us/institution/smithsonian-national-museum/id393005739