I've included music into my game as a .ogg format, and I have it so whenever a boss is visible, it stops the current music and plays that one. However, for both songs, they play basically every tick even though I didn't use that option.
"Every tick" is basically a blank condition, all events that are true will happen every tick. So your boss visibility check is going to happen every tick while the condition is true. A solution to this is to add another condition to the event, under system object choose "trigger once while true".