Volume down doesn't work

@yakapo7 if you have checked all settings and software and it still is not working, my prime suspect would be a failed power/volume button flex cable. Since I can't find a specific guides or video for this, and iFixit doesn't yet have a specific guide for that, I suggest you check videos like this (it mention the volume flex at 4:00min). It is always possible to post images if you get stuck anywhere or have questions about anything, with your Question. Use this guide Adicionar imagens a uma pergunta já existente for that.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!