So I was wondering, If I play a sound from the server, if a player joined in the middle of the song, would it play from the start or from the middle, or would it play at all?
It depends on how you script it and where the sound object is.
If the sound object is inside of the player, the sound will be local to that user only. If the sound object is played from Workspace, the music would be heard at the same time by everyone no matter if they join later/sooner.