Bah, I had two instances of a global XAudio2, removing one seemed to fix the muffle, but there's still a slighty echoing effect to all tracks. Not the exact same as my example MP3, but playing the files back-to-back you can still hear a slight reverb from Construct.
The current echo side-effect makes the feel of music change quite a lot, so I hope it's fixable somehow... It's the difference of making a studio recording and making one in a big empty room.
The music plays on a looping reserved channel which was made reserved at start of layout.