“You wouldn’t download a car” arguments aside - what is the ethical boundary for “They’ve made enough money - I’m entitled to enjoy it for free” ?

I agree, that's the kernel of the issue here.

TL;DR: I'm curious if there's a way to make effective ethical arguments when people have fundamentally different world views, including whether or not morals have any solid grounding.

You can judge a moral system by how society would evolve if everyone followed it.

If nobody pirated anything then we would still be buying music for 99 cents a track (or more) and music streaming would have been quashed by the labels.

On the other hand if everybody pirated everything then the music industry would have collapsed.

I conclude that the world needs all sorts of people.