This is an excellent point and an issue that is pretty pervasive, both for hosted and self-hosted blogs.

I don't know what the answer is. I think hoping that people will back up their audio / video is unlikely. But archive.org is pretty incredible, and I also like that a project BDFL like Matt has acquired some WordPress properties for the purpose off keeping them online as well.

I know Siobhan McKeown has struggled with some of this issue quite a bit while she's been working on her book. She's be a really good person to ask about it.