The feature in question is the ability to boost the audio output from the speakers, as now recent iPhones already have stereo speakers and are also loud enough, but if you feel it would have been better if the volume was a little louder, the equalizer setting Simple can do it for you.
But it should be noted that changing the equalizer setting only enhances the sound from the software, but if the speakers are blocked by dust or damaged, you may get a boosted output.
Steps to Increase iPhone Speaker Output
1. Open your iPhone and tap Settings
2. Open Music → Eq → Tap on the “Late Night” option
3. Go back and turn off the Sound Check . option
Now you can go back to your music player or any media app and start playing the media file, and you will notice a slight increase in the audio output.