Of course, the copyright aspects of this are another matter. In general, if you don't have the distribution rights to the custom music, then doing ANY of the above is a copyright violation for which you will pay the consequences. So, I don't recommend doing this at all unless the music is public domain or you wrote it yourself and don't mind giving it away.
I am not a lawyer, but using copyrighted music is not something you can get in trouble for if you're just an individual making a creative project. It's fair use.
We (the players) aren't selling the parks or rides, we're simply sharing creative projects that aren't any different from, say, a collage that has a few copyrighted photos. The only difference is that our "collages" are in a different medium.
Even when it comes to sampling, you cannot be sued for using unauthorized samples unless you are actually selling the songs that have the samples in them. Musicians use uncleared samples all the time for mixtapes, tracks uploaded to Youtube or just given away free online, etc. They're not making money on those songs, so they can claim fair use. Even if someone litigious owns the copyright, they can't sue for proceeds because there are no proceeds.
The problem, as I understand it, would be for Frontier. They don't want to be seen as providing a platform for people to illegally share copyrighted material, which is probably part of the reason we can't upload our own GIFs or JPGs for building textures or whatever. (Along with the fact that it would probably result in quite a few terrible-looking pieces. I can just picture a warped, pixelated "billboard" in hanging on the side of a building in someone's city. Or a city of buildings wallpapered in Miley Cyrus.)
If they do eventually allow us to include our own music and sound effects in Workshop/shared files, there would be some sort of disclaimer and/or warning that we can only use open-license audio content, or content we make specifically ourselves. Regardless, it would be very difficult for anyone to argue that players are buying a $40 game and construct and share elaborate theme parks just to trade copies of the latest Nickeback track.