Well, they put all that work in, and added new tracks. We're gonna have real issues with any sort of music added.. copyrights will probably prevent most things we'd want to add to be, you know, added. (Unless we could claim fair use, but that is very iffy). So, a lot of royalty-free songs, and possibly the stuff in the game already (although I'm not sure about that, we'd still have to pull it out, edit it, and put it back in, but it's from the game and the game has the rights..)
Plus, well, some of the DLC is mostly for the console users anyways, that or the people who don't bother with mods. The clothing packs and weapon packs are neat, but while it sells I don't know how many of the people here actually bought the 'executive privilege' cheat pack.
Our best bet is to come up with either a tool or a good guide on how to insert your own music. That way there is no royalty issue as the consumer already owns the song.