The concept of "selling out" requires you to have some core values which you and your audience share. If you're a hard rock band and you make a cringe disco album because that's what the record label told you to do, that could be seen as selling out. If you're an anarchist crust punk and you get signed to a big label that could be selling out. If you're an underground DJ and you do the soundtrack for a big movie that could be selling out.

I don't think most music artists have the necessary relationship with their audience to "sell out", because their music isn't ideological and they don't have a real relationship with their fans. As famous sell-out Laura Jane Grace sang, the content is so easily attainable that the culture is disposable.