Try a different format for the sound. See if that will make a difference since it's not a script problem. Or maybe see if it's a problem on a different computer too. I'd try everything I could think of and if it's not relative to one PC, check the script at every point where the volume could change. I had a similar problem where I had an if statement, that constantly changed the volume lower. Thus creating an effect that made it seem like it would cut out after turning it up louder at one point.