I GM with a soundtrack for 2 point something years, closer to three actually. I think that my style changed within those years, and... There are 5 tracks that I think that are something else entirely, level and all, tracks that changed the way I GM from the moment that I've heard them or used them.
Today, I've decided that it's the time to share them. I don't know if this post will be useful, but I hope that through this post you'll be able to decide better about whether GMing with a soundtrack can be useful to you, is good to you, or maybe that it doesn't.
It all started about 3 years ago; when I first gave a thorough listen to the famous theme of the X Files. At the time, I was GMing mainly D&D and I've decided to make a switch in style from dungeon crawls to something else. I didn't know at the time what will it be, I only knew that I want to change something. This listen changed about everything. It was the moment that I've decided 2 very important things for the rest of my GMing career: The first was that I'll GM mainly with soundtracks. The second was that I'll try to break out of the dungeon crawls to the darker places. It started small, with the little addition of moral dilemmas, but from there it grew bigger and bigger till today.
After this decision, I've found coincidentally the great site of Radio Rivendell. At the time, it was all I had, and I've went through all of the links from there to free music. Then, one day while I listened to this wonderful radio, came this theme. Needless to say, I was immediately hooked. I've listened to it about once or twice (or sometimes even trice) a day, looking forward to the point where I'll be able to use it. So simple, yet utterly powerful and evocative, it was another point where I've decided to stretch myself. This time, it was a little bit back into the light. Another thing that came to me from so many listens and thoughts about how to add it was the addition of dream sequences and party sequences to my games. Not regular modern parties, but more of the ball type of parties.
Not a long time passed, and I've found a band that created the exact music I was looking for, their name was Midnight Syndicate through this theme. Simple yet powerful, it was the point where I've decided 2 things. First: I'm going for horror games. Not a long afterwards came the second thing: I've can GM from one album. Until then, you see, I've had 4 CDs, which I switched when I needed a different emotion. It was this album that made me understand the power of not switching albums. I've still switched tracks all the time, but it was only from one album. You see, it was the first time I've connected albums to games.
Then came another click. I was asked to create a particular soundtrack for a Twilight game. As any sane person would tell you, I've went reluctantly into the work. It was a bad movie series, and I was yet to find any merits to it, but if I'm asked to do something, I try to do it. I went to devour the soundtrack album, and I've found the first merit. Too bad it was the only one. A theme so lonely, so sad, and my ideas started to roll, and not for that game, but to others. As my players will tell you, it was this theme that played when the characters killed the players or freed them.
A few months afterwards, when my AFI 123 project began, I've discovered again the soundtrack of Psycho. It was the first track that caught my ear. It was the first time for 2 things: The first was of using (almost) solely soundtracks from movies and not making soundtracks from both movies and ambients. The second thing was far more important: It was the first time that I've understood that I can get from soundtracks essences of stories, and that if I'll learn how to GM according to the movie soundtracks (and to make the games according to them) I'll be able to stop switching tracks in the CDs and get to the point where my players tell me that it's like a kind of magic. There's a trick to it that I've learnt from watching the movie: If you know to improvise and you know the soundtrack well enough, and the soundtrack essence is close enough to the story, your brain will make the connections for you between the story, game and soundtrack. When there wasn't a good enough connection, I've learnt how to pace things to get what I want. Although it wasn't the first time that I've GMed with a soundtrack for real time, adjusting myself to the flow, it was the first time that it was for more than 2-3 scenes.