Duplicate of https://bugs.mojang.com/browse/MC-120952, which is marked as resolved, but it's not.

Setting speech volume in Audio settings does not change volume of the Narrator. Narrator is ALWAYS at 100% volume.

Setting a different voice in the old Windows narrator settings (accessible through the old control panel > Speech recognition > Advanced speech options > Text-to-speech) does change the voice, but there are no volume controls.