It’s not just musicians, it’s also little websites like mine, for this same reason:
When someone buys digital music from an artist directly they’ll see long, slow downloads that hopefully manage to finish. When they stream music from that same musician’s site it’ll hang and pause unless it’s compressed to hell. But when that same person buys from iTunes? Smooth like butter.
Yes, it will be much more obvious for audio and video when it comes to slow download speeds, but it could also effect every independent website out there as well. I’m not going to pay out money to every ISP out there to guarantee my website isn’t slowed down, or even available in the various search tools being used by those ISPs.
It would very much be like Google asking for an ad fee for every website in the world to be indexed by their search tool. That would be awful. Net Neutrality is what prevents your ISP from basically doing the same thing, putting small up and coming sites in a ghetto, far away from the high-speed quality that major players can afford to pay for. How does that help you as a consumer?