Are there any specific guidelines for what constitutes fair use of an audio sample? For example, this page details guidelines for educators, while this page at illegal-art.org describes some famous modern cases involving infringement of music copyright. The educator guidelines list a “30 seconds or 10%, whichever is less” rule for sampling music.
The application that I’m interested in is posting small music samples on a website that people could download for their computer startup sounds, for example. I think there are tons of fun rock riffs out there that make a badass way to start up a computer, but of course I don’t want to get into legal trouble. (I don’t really know who I’m kidding, I’m sure record lawyers would attack this site like sharks on a bucket of chum.)
I could definitely stick to the 30 second/10% rule, and it would be not-for-profit. Anyone have any (hopefully optimistic) opinions on this proposed endeavor? And yeah, I know you’re not lawyers, most of you at least. 
After The Gray Album* came out on the P2P networks, there was a long article in the L.A. Times about how the big record company who owns the Beatles music was really angry about it. There were interviews from several other artists, who’s reactions varied from “Sampling anything at all is absolutely wrong” to “As long as it’s not, say, a clip of Hitler yelling, with my voice in the background, or some other offensive audio, I really don’t mind.” (both quotes highly paraphrased) IIRC, it wasn’t entirely clear what’s legal and what isn’t. However, that was mostly in response to sampling like “illegal arts” does: mixing tracks from two or more different songs together, creating what’s called “bootlegs” or “mashups.” This is entirely different from what you’re doing.
As for whether your suggested activity would be legal, I don’t know, and I doubt that it’s a simple question. OTOH, if you’re not selling it, I’m sure nobody would mind very much. Notice that (AFAIK) nobody has come after “illegal arts” or most of the several other mashup artists that you can find on the internet. They have bigger problems to worry about, like the Gray Album.
*Uses samples from The Beatles’ “White Album” and Jay-Z’s “Black Album”