When I change the sound, there is a threshold of 13% where I cannot lower the volume any further without losing it. I tested this with the Shortcuts app: when I put it down to 12%, you hear nothing. Thus, there is still some space left that I can shift the volume button around. That small space leaves all volume at 0 volume. In a quiet room or when you have a noise cancelling ear muffs on top of (in-)ear phones, the minimum volume is still too loud.

I have an iPhone 7 with iOS 15.7, but this should be needless to say since this problem has always been there.

One workaround is to put in some ear-plugs, use headphones on top of them, and then just put the sound-volume slightly louder than the minimum just to hear enough, but that does not work for (in-)ear headphones. What I am searching for is an App or setting that allows changing the the system volume finer than the OS menu (= the buttons of the headphones) allows it, and below the threshold of 13 %.

When I do not use headphones, but the normal loudspeaker, I can go down to whatever percentage and it will always react. Therefore, it is just a problem of the headset. The threshold is 13% both for a Bluetooth headset and a cable headset.

In some way, this not a duplicate of Can I set the volume on my iPhone lower than the slider goes? since that is about a missing fine-tuning, while I cannot even tune anything in rough 1% steps.

How to get a lower sound-volume than the system minimum of 13% volume for headsets on an iPhone?