Terasology is a free, open-source sandbox game heavily inspired by the popular Minecraft game. It gradually became a platform for all sorts of gameplay settings in a voxel world.
Terasology has been build from the ground up to be modular and extensible game. You can found numerous modules on their Github page.
The music, composed by Chris Köbke, is also available under the CC BY 4.0 licence. You may get volume 1 and volume 2 directly on the Github account, or listen to it on SoundCloud .