I hope I've used the right terms in the title; I'm not a native speaker.
The singular of media is of course medium. So when talking about one medium's content, the genitive s and apostrophe are easy enough to use.
Late edit for clarification: I mean media as in recording media. A CD, a cassette, a book are all media. The correct singular, I guess, is medium, although media seems to have become acceptable as well.
How about three media? The three medias' content? The media's content? I guess I could use the latin genitive, but that would probably be wrong and make me sound like a smartass.
Or even Media'?
Or just cop out and say mediums'? That doesn't sound better to my ear, and I'm also talking about information containers,not seers.