[Closi]

Like you say, the law is not settled on this, but I assume if the author got a takedown request they would probably comply.

In many instances a policy of "ask for forgiveness rather than permission" can get you further, faster. While Nickelodeon are unlikely to grant you a license to the Spongebob voice because that has broader licensing and IP repercussions, they are likely to tolerate a research project using their characters (e.g. just as they have to-date tolerated The SpongeBob SquarePants Movie Rehydrated, which was a fan re-creation of one of their actual movies).