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 iFixitでの質問に画像を追加する方法 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!