I bet Toady could write and add some more music to the game within 3 days. Or, he could just buy the rights to use another person's music in Dwarf Fortress, and have it over with in a matter of hours. And like nomnum said, just because there's an addon that fixes a problem doesn't mean the default Dwarf Fortress shouldn't get it fixed as well.
Have you ever made music yourself?
I have, so all of this is coming from personal experience:
Yes, writing and eventually making music is easy process. The difficult thing is to actually make it sound good. Anybody can write some symbols to paper, play some strings from guitar or push some buttons from computer keyboard and you have music. But does it actually sound good? That's a different matter. So, few days might be accurate if
A) Toady has more experience in making music
B) We are willing to take the risk that the music is boring or even bad at best.
Yes, Toady could buy rights to music made by others. The problem is that, if one wants some quality, it costs. And with current monthly revenue, I'm not exactly sure if Toady is interested in investing to such thing right now. Of course, one could get lucky and find a talented person who is willing to make the music cheaply or even for free, but, outside the community, the chances are slim enough so that Toady shouldn't count for that.
Also, there are some free music/sound available in internet. Toady could use those as placeholders, but then again it would take some time from him to actually go through all those free sounds to filter out the crap and find the ones worthy to the game.
So, implementing more music or sound to the game is not a huge task in programming side. The problem comes with making the actual music/sound which has good enough quality. I bet that nobody here wants some bad music just because there needs to be some...
And no, I'm not "shooting down" the idea, in my opinion there should be more music and sound. But currently there are, IMO, more important things (features, bug fixes) to spend time on...