It's confirmed, you can't trust every review you read – Volume Master - Dial in the sound of your music player with a control booster Review

The app does provide finer and more incremental volume control. Just look at the negative reviews-- why doesn't it make my iPhone louder? Why doesn't improve my bass? Why? Because the app doesn't promise to do that. It does what it promises to do. Relative to the one reviewer that commented they saw the iphone default volume slider and that this meant it did nothing, that's false. The slider shows whenever the volume changes in any increment.
Review by TechClock on Volume Master - Dial in the sound of your music player with a control booster.