I am trying to figure out how to make this music sound muffled, but I need a certain song, but I just need to have to sound like maybe that you are standing outside of a wall with loud music.
Adding a low pass filter seems the first thing you need to do to achieve this effect.
Also convolution reverb using an impact that fits your desired application (courtyard, outside building blocks, hall-way etc.) might help emulating realistic sound reflections. It is a common technique for video-games.