http://www.gamefaqs.com/boards/620272-playstation-vita/62028667?page=1
Here's a thread where one of the guys a few posts in had the exact same problem. Now that i think about it, the demo's sound cut out on me, too. I didn't really care because I was at work and the guy next to me was watching a movie. Anyway, I'm not alone. Really, I gues s the problem is with the Unit 13 demo and nothing to do with the Vita or its memory cards.