Hello djsf!
Thats a complex question, I'll try to get close to an answer.
At first, our mission is to strengthen free music, the free as in free speech. This means our aim is to support musicians who are using free licenses and we are totally against any DRM. That being said, due to the federated nature of Funkwhale, its pretty hard to enforce that access control, at least I have no simple solution in mind. The model I have in mind is bandcamp, where you are allowed to stream track a few times until you get prompted to buy the music. If we'd want to do something similar, the tracks would already be federated to remote instances and we basically loose control of what the admin of this instance is going to do with it. The same is true if one user of an instance "bought" the track, the admin gets access and we loose control.
On the other hand, as soon as we start to interact with money and payment providers, we open another box of potential issues. This is totally not an argument against implementing it, its just that I don't think we have the experience or developer power to jump into this. We would need to evaluate this.
However, in the past members of the Funkwhale Collective already considered potential solutions for the problem "How to fund artists" and the current idea is available as a prototype at https://retribute.me/
This doesn't really restrict access to the music, yet. But the idea is to have a external, eventually distributed service that is able to connect several funding platforms and Funkwhale or other Fediverse platforms. I'd love to spend more time on this topic in the future, maybe there is potential to get closer to the point where we can put high incentives to the listener to fund the artist. Again, I don't think enforcing access control is something we can do. But maybe we can get closer to what Bandcamp does.