How would I make it so when people are in-game they hear music in the background?
If it needs explaining more ask.
Well there is an addon called PlayX that could help you with that.
It plays YouTube videos through a screen or a radio, so you could put a song from YT and it would play it for everybody who had enabled it.
Here’s the addon http://steamcommunity.com/sharedfiles/filedetails/?id=106516163
You can configure it through the Q menu