I'd be careful, I think that in the way we are using things it would fall under 'fair use', but I (and I think this was what SO was getting at) would not like to be taking chances. There is plenty of CC music out there, especially for short clips.
Maybe a solution is to have an alternate mod-mod plugin/add-on pack(in mod-mod section of course) that is a less copyright friendly (but not illegal) as add-in folders or a collective package. This was done in the Planetfall mod by Maniac as instructions to allow people to bring in actual Alpha Centauri assets into the mod (if you owned a copy of the game if course). I think we should have a thread in the mod-mod section will allow those recommend resources(for those who own them) to at least be listed (by those want to) and all the grey or controversial stuff parked in that thread, until the grey is sorted out. This way the material will not get thrown away, and not risk the mod, yet still get considered. At least this way ideas will not get lost, forgetton, or unnecessarily avoided while a line of what is appropriate can be drawn. At the very least it would be a good place for discussion of what could be legally put in C2C.
Anything that brings concern of copyright, should probably put in 'quarantine' in the mod-mod.
Note: using idea material from popular games, movies, etc, should not trigger issues, assets directly takien woukd ;pics, sounds, soundtracks, etc. (example: 3d model of Enterprise from Star trek video game) would be an issue, but copying the ideas from the tech-tree, transporters, tricorders, galactic federation, even names ( like Vulcans, Klingons, etc.) , all the way up to details of stories.
Notice: the content and ideas of most Star Trek, Star Wars, Bablyon 5, and other Sci-fi games/movies/etc, and use of ideas from other games should be legal so long as it not copied directly (files ported) or sold. Use of the information should be fine, so stories, tech-tree, lists of ideas, locations, names, locations, even plot should be fine. Mods of poplular works shoulf be legal if you don't directly copy. This means you can Star Trek or Star Wars ideas in C2C but not videos clips, screen captures, or music. You are allowed to make a complete Star Trek mod with names, units, plotlines, and stories from the show and movies.
Parts from other mods are notcopyrighted but please politely ask (if possible); civfanactics considers all mods freely usable. The ideas are available for everyone.
This is just my understanding of 'fair use'. (note I have a degree in business admin and have made an 'A' in my 1 business law class, not that that makes me more than somewhat knowledgable.)
Simply separating the more questionable material and not making it an 'official' part of the mod, would probably help your concerns.
I would check things out before I would hesitate too much.
I also agree with being careful, but not paralyzed.
Let's not limit ourselves (more than reasonable).
I hope C2C explores as much as possible.