Adjusting Boss Music Volume Without Muting Other Game Sounds

The boss themes are routed through Minecraft's music channel, so you can balance them against your other audio instead of living with the default mix.

Which Slider Controls It

Open Options, then Music & Sound Options. The boss themes respond to the Jukebox/Note Blocks slider, not the main Music slider. Lowering that slider reduces the boss tracks while leaving ambient music, mob sounds, and blocks untouched. If you want the themes loud and everything else quiet, raise Jukebox/Note Blocks and lower the rest.

Testing Your Mix

Load into a world and start a boss fight, or use a command to summon one in a safe area so you can hear the theme without dying. Adjust the slider from the pause menu while the track plays so you can hear the change in real time. Note that the mod fades music in at fight start, so give it a moment before judging the level.

If You Hear Nothing

Check that the correct theme is toggled on with its trigger command, and confirm the Jukebox/Note Blocks slider is not at zero. Also check that no other audio mod is overriding the music channel, since overlapping music managers can cancel each other out.