The music add-on mod can be installed individually without the base Calamity Mod, and it will contain all of the music boxes and tracks, in case the player wishes to listen to Calamity music in-game without having the full mod installed. The music boxes will be unobtainable, however, requiring the use of cheats to acquire.
Calamity music boxes cannot be activated with wires in multiplayer.
The Boss Rush Tier 1 Music Box is based on the Terminus, which is used to start the event, the Boss Rush Tier 4 Music Box is based on the Calamity Title Theme Music Box, and the Tier 5 Music Box is based on the Lower Abyss Music Box.
The "Acidic Downpour" music box is craftable because its track only plays before Polterghast has been defeated and is therefore permanently unplayable if the player defeated the boss before recording it.
The "Void" music box could possibly be a reference to the game Hollow Knight, In which there is an area called the Abyss, containing a large black object.