I highly suggest reading into the project first. The Wiki article I linked before (https://en.wikipedia.org/wiki/15.ai) answers all of your questions about copyright infringement.
In terms of copyright infringement, your wiki link answers nothing. A court ruled that Google could use copyrighted book text to train an algorithm to improve search results because the copying was highly transformative and did not serve as a market substitute to the original work.
Meanwhile 15.ai is using copyrighted voice recordings to train an algorithm to synthesize new voice recordings that sound like they came from the original speaker. This is radically different from the Google case. Just because one instance of using copyrighted material to train an algorithm qualifies as fair use does not mean that all use of copyright material to train any algorithm also qualifies as fair use.
There is absolutely nothing about this that is settled law. In the next 20 years there are going to be lots of lawsuits, lots of settlements, possibly a few rulings, and maybe even a few new laws. I find the whole topic very interesting. YMMV.
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).