I don't think that these limitations are fair. At least for the music ones. 30 seconds does give you enough time to get the full effect of most songs. Think about these protest songs, most of them have a much deeper meaning, and the students would benefit more by hearing the more of the song in a presentation. I feel that as long as you site the song, and make it clear that it is not your work you should be able to use as long of the song as you want. Also going along with books, poems, and pictures. I feel that as long as you obtain the material legally and aren't trying to sell or pass off the material as your own you should be able to use as much as you need. By putting a limit on this amount or time you can use, also in a way puts a limit on how in depth you can go when talking about the topic.