TODO:
- add more methods for playing sounds
- normalize sound files
- add ambiance registration for biomes
- add option to get sound by string name in "SoundGroup:get" using string.match
- allow SoundGroup arithmetic expression using string
- create SoundGroup instances for all cached mod sounds
- fix "node_dug_meta" & "node_place_metal" are the same
- add more sounds:
	- hit/damage
	- tree
	- car
	- motorcycle
	- train
	- chalk
	- generator
	- horn
- add sounds from:
	- cmer
	- cmer_zombie