All players need to have YPlayBoombox installed and be in the lobby when you enter the URL, or they will not hear anything when they join the server.

If you are currently playing audio and want a player who just joined to hear it too, you can enter the URL again or use the /yp history command. There is also a resync feature to automate this process for the user if they wish, but it's not guaranteed to accurately sync the playback time if the boombox is already playing.