Custom Music makes a project have more atmosphere. There are 2 ways of doing this.
Way 1
Find the sprite "Custom Music" and then upload music of your choosing
Way 2
I prefer this way better. Go to the backdrop and then upload your music. Then check the ID of the music, then set c.music to the music ID and then BAM! Custom music! I prefer this way because you can have multiple custom songs AND you can change the song!