So after watching to all the available presentations of MKX at E3 I could find, I started trying to listen specifically to the music. It's down pretty low, especially with people talking over the video, so it's not easy to make out much of anything. But the few times I could make something out, I heard what sounded a lot like this track from MK4, interestingly enough.
riptaccess="always" allowfullscreen="true">
I have no idea if that is the same track, but it sounds pretty close. It also sounds like all the stages are using the same music right now, so NRS very well may have been using placeholder for all the music.
Only time will tell for sure on any of this, of course, but please post your thoughts about what little music can be heard in the demos, and what you'd like to hear in the final game. Would you like to hear music similar in style to MK4? DA-Armageddon? Classic stuff?