Oh crap. I was just forced to remove soundimage.org from the list. It looked like they were using CC BY 3.0 but then I read the actual license text and noticed they actually add a very sneaky teeny-tiny modification to the license text.
2. Restrictions
The use of the Licensed Material in media that violates Youtube community guidelines is prohibited.
This license applies to all files … Sigh. Such a shame. It would have been such a wonderful site. The YouTube community guidelines are pretty terrible, adding this to your license is horrifying (even from a non FOSS-zealot point of view), plus, they are ever changing, making a legal use very difficult. From what I read, apparently the author added this restriction because they don't want their work to appear in “obscene” material … >_>
I always hate it when people add little pointless restrictions in their licenses that serve absolutely nobody (not even themselves). >:-(
Whatever. Thankfully, I never used any of these files from this site.