TODO:
- add more methods for playing sounds
- rename sounds (e.g. "place_metal" to "metal_place"
- normalize sound files
- document node sound groups
- 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
- add more sounds:
	- hit/damage
	- tool breaks
	- lava cool
	- wind
	- tree
	- leaves
	- dolphin
	- leopard
	- lamb
	- squirrel
	- airplane
	- car
	- motorcycle
	- bicycle
	- fire
	- raccoon
	- train
	- rain
	- whale
	- inflate
	- pop
	- gas motor / generator
	- chalk
	- horn
	- creak
	- door open
	- door close
	- door slam
	- swan / goose
	- snake hiss
	- snake rattle
	- seagull
- add sounds from:
	- cmer
	- slingshot