I was looking back at some very old work I have done in the past when I was just starting out in the working world and one item was actually an audio book that I helped to create. Usually as a talent you should expect maybe some kind of royalty as an example for these kinds of projects. But as I was starting out I was just happy to have the opportunity to work on anything. It reminds me how other artists sign contracts that literally give say a photographer full rights to use their likeness in a picture which is going to be listed at stock photography site. Like there, usually for professional work there would be expiration periods or arrangements of royalties.
Now usually people say you should always charge your worth as an example or never do things for free if you want to be considered as a professional. But sometimes getting that initial experience can be more valuable to give you the necessary push to work on lucrative contracts. I still remember some famous stories of people like the creator of Star Wars giving up the rights to his film in exchange for having full licensing rights when it comes to merchandise for the franchise. That wasn’t a bad tradeoff financially for a person who needed the opportunity too.
I guess you can treat this as an important business exercise early on where you have to weigh in the pros and cons of surrendering your rights just to get some work experience to enable you to pursue those bigger opportunities in many ways. No different than potentially giving up a large portion of your business to an investor in order to grow.