With "play music", the xAudio2 plugin pauses to reload the file -- the so-called "loop music" is more of a "start over as soon as the computer feels that now would be a nice time for it".
You can "load resource to channel (+loop)" and "play channel" for gapless looping, but that won't work with an mp3 file.
In any case, mp3 is a bad format for gapless looping: because it was originally designed for video soundtracks, it only supports lengths of multiples of ... 1/24 second or some such video frame rate. If you encode a sound of any other length to mp3, the end is padded with silence to the next frame boundary; so even with perfectly gapless looping, most mp3 files you can produce will have a gap encoded right into the sound data.
In summary: sorry, I can't solve your problem unless you can make a _really short_ loop, save it as .wav and use it as a "sound" rather than "music".