You can create two variables - SoundsVolume and MusicVolume. When starting playing any sound, set its volume to SoundsVolume. When starting playing music - set its volume to MusicVolume.
So when the sliders are changed, you save new sound/music volume in these variables, and can also set music volume for currently playing music.
If you don't need to play several music tracks at once, it's OK to have one tag for all music in the game. But of course for different sounds it's recommended to have different tags.