You might have noticed that your iPhone shows a useful playback controls widget on the lock screen whenever you're listening ...

Under this project, the company was supposed to be testing a haptic volume control ... to the iPhone, suggests the source. Apple was testing this prototype even before the iPhone 13 series launch ...

labeled Always Show Volume Control. Enable the switch, and you'll be able to adjust volume from the Lock Screen without having to unlock your iPhone or use the physical volume buttons. Elsewhere ...