The bug

When a sound is defined with a category of music in the sound definitions file, this has no effect. The music slider in sound settings does not affect the volume of the music.

How to reproduce

1. Download the attached world
2. Press the button on the command block
3. Change your music slider setting as the Ode to Joy plays
→ Volume is unaffected

Sound definitions file in the bundled resource pack is duplicated here for convenience:

{
   "music.test_music": {
      "category": "music",
      "sounds": [
         {
            "name": "sounds/music",
            "stream": true,
            "volume": 2
         }
      ]
   }
}