Boss

Good question

and do different bosses hear different player music when someone else is fighting them

very true

no one knows

The boss probably hears the theme song of the game. Take Bowser as an example, when Mario approaches him, he hears the DU-DUM DUM! DU-DAM DAM! *bang*

Canonically no, the game provides intense music so the player will get pumped and scared for the boss, but in the game's canon, the characters hear nothing but silence

will yes but it's intensified far more beyond than what the player hears because the player is more dangerous than the boss

one of the worlds 7 wonders

Mega man X Boss Music kicks in

Maybe bosses try to kill us because they hear horrible player music while we get awesome boss music

this is so true why

nice

The boss:

when you beated the boss for the 1000th time