The volume sliders don't work if the /playsound volume is set too high. If volume is set to 1000 for example then the volume of the sound playing will be at 100% unless the player turns off the sound category or adjusts the master volume.

For example:
Turn on voice/speech volume to 100%
Run this - /playsound minecraft:entity.ghast.ambient voice @a ~ ~ ~ 1000
Turn the voice/speech volume to 0%
Run this again and observe the volume didn't change - /playsound minecraft:entity.ghast.ambient voice @a ~ ~ ~ 1000
Turn the voice/speech volume to off
Run that command again and you shouldn't hear anything.

This can break custom music in maps if this is intended as music needs to be set to a high amount of volume so player's can continue to hear it no matter where they are. With this bug player's can't adjust the volume of such things without having to adjust every other sound in the process too.