I feel what you're going through so strongly. My 2nd year I got into res with a bunch of my friends and one stranger who was assigned next door. Like your roommate, she played thumpy music to get herself motivated for class early in the morning, for God knows what else during the day, and for bed at night. Sometimes she would play the music for eight or ten hours straight (eventually I became so frustrated that counting in disbelief was the only thing I could think to do). This was at a time when I was applying to the Creative Writing program here at UBC and was spending long nights making and polishing my manuscript.
The one thing I can recommend is not to push it with your roommate if you encounter resistance. I think even the tiny pressure applied by a gentle request to turn the music down can provoke obstinancy in some people, and in my case outright hostility. I was sucked into a pretty negative and aggressive spiral that involved, worst of all things, angry letters, and all it did was exacerbate the issue until it completely consumed my mind. Your roommate will tell everyone she knows nasty things about you if she perceives you as spoiling her fun; she will buy a newer, bigger stereo system at Christmas.
I don't really have a solution for you. Like you, moving out wasn't an option because of the hassle during the busy school year (and I had friends there, also), but mediation was not really an option either because of the drain it would take to enforce. Earplugs can be painful, headphones inadequate, and leaving for elsewhere humiliating. In the end, I simply had to adapt as best I could, with thicker skin, and weather it until the end of the year.
I can still never quite believe that there are people out there who do not try to understand others. At least be sure that you're in the right. What it comes down to is that no one should willingly engage in an activity that makes someone else's life unhappy.