But selling ingame music as a separate DLC ???
Well, it's definitely not the first time it's happened. As an example, on consoles, Dynasty Warriors 7 has a handful of music packs and weapon skins as DLC for a couple bucks each (the packs usually contain 3-4 tracks, I believe).
I can see why it wouldn't be the best use of paid DLC, but it's definitely not the worst use of it I've seen. And so long as it's fairly cheap, I have no grudges against them doing it in moderation.