Hi there TuomasEi,
We understand you're experiencing an issue with the volume on your headphones with your iPhone, and we'd like to share a resource that may help.
Check it out here: Adjust the volume on iPhone
Specifically, take a look at the section below:
"Limit the headphone volume
You can limit the maximum headphone volume for music and videos.
- Go to Settings.
- Tap Sounds & Haptics (on supported models) or Sounds (on other iPhone models).
- Tap Reduce Loud Sounds, turn on Reduce Loud Sounds, then drag the slider to choose the maximum decibel level for headphone audio.
Note: If you have Screen Time turned on in Settings, you can prevent changes to the maximum headphone volume. Go to Settings > Screen Time > Content & Privacy Restrictions > Reduce Loud Sounds, then select Don’t Allow."
Cheers!