I´ve always been wondering why developers don´t include such a simple option as custom soundtrack.
It isn't down the developers, it's because of the hardware. The same problem that stops us from having cross game chat also prevents us from listening to our own music while playing a game.
Go into settings, mute the music. Then proceed to play your own music from a laptop or stereo.
Don't be a smartass. People like to feel like the music is coming from the game, not that its taking it over. Why would people not want to hear the sound effects?