Play a track. Whatever is playing fades out over <paramref name="fade" /> seconds while this fades in. <paramref name="volume" /> is for this track only, on top of Sandbox.Game.Music.Volume - use it to tame a loud file. Looping is gapless if the sound has loop points (tick Loop in the sound editor), otherwise the track restarts when it ends. Playing the track that's already playing does nothing.