This module uses 4 components:
Real instruments -- the physical manifestation of the instrument, the one that is placed in the world
Virtual instruments -- the definition of an instrument, that is, what samples it uses, name and category
Sound player -- used to apply modification to all sounds emitted. Represents the actual source of sound (Right now it does little, but if you want to make certain factors
further modify sound more you'd start from here)
Synthesized Song - just like /datum/song it's what parses and plays a melody
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Real instruments inherit from either /obj/structure/synthesized_instrument (static musical instrument) or /obj/item/device/synthesized_instrument (handheld musical instrument)
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Virtual instruments all inherit from /datum/instrument. 
Virtual instruments should follow this structure: /datum/instrument/category_name/instrument_name.

Refer to any file in ./instrument_data to see how it's done.
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Sound player inherits from /datum/sound_player
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For synthesized song only use /datum/synthesized_song
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