Depends on where you want it to start. If you want to start the music as soon as someone joins the server, use PlayAudioStreamForPlayer or PlayerPlaySound, under OnPlayerConnect.
If you want to start it at any other point in the joining process, just use one of the 2 function provided above in the callback of your liking.
Then under OnPlayerSpawn add either StopAudioStreamForPlayer, or if you used the other function use PlayerPlaySound, and add 1 to the sound id (if you was using i.e id 1009 for the music, use 1010 to stop the music).