Create an blueprint [it can be an empty blueprint or a drawing blueprint], go to "edit blueprint", scroll down until you found the "music" section, click in the "+" icon and create your music :3 [Or if you wanna play a file of your choise, you can just go to "rules", put the "when this is created", then scroll down to "play a sound", choose "from my files" and download the file [the sound must be 30 seconds long at maximum]