SoundtrackPlayer

Base command:

/soundtrack <play/stop>

/soundtrack <play> <id> <"sound"> <volume> <fadeinTicks> <fadeoutTicks> <loop> <pitch>

/sountrack <stop> <id>

With this mod you can add songs from resource packs or Minecraft natively. If you leave the server, the song will continue playing at a reduced volume, and when you rejoin, it will resume as if you had never left.

You can also change the volume in real time without affecting the audio if you use the same command with a different volume.