Origins of various assets:

metal_floor.jpg: Free texture from http://cgtextures.com/texview.php?id=22804
lava_bg.png: Created by Cameron Desrochers using Gimp's Render->Lava menu item
soccer_ball: Free model from: http://www.turbosquid.com/3d-models/free-old-balls-3d-model/671224
crate: Free model from: http://www.turbosquid.com/3d-models/free-3ds-model-wooden-crate/597590
music: Free short clip from http://www.playonloop.com/2010-music-loops/tunnels/
music2: Free short clip from http://www.playonloop.com/2011-music-loops/pop-corn/