Buildbox has made the implementation of music and sound effects super easy. It's a lot like replacing graphics on assets. You just drag your audio file (an MP3) onto the speaker icon in the properties field of whatever you'd like to assign it to. Let's get to it.
There are lots of ways and philosophies for doing this. Each UI can have its own background music. It's important to know that with Buildbox, the music will keep playing until it is overridden by another piece of music. What does this mean? It means that if you set a piece of music to play, it will continue playing across all UIs unless another UI that pops up has its own music. For instance, menu music will continue to restart on each menu you go to (if each menu has music assigned). Also, if you assign music to a world...it will restart every time the player restarts the level.
This can get tiresome, and just downright annoying to a player. So what I do is create one...