After installing the ios 11 update users are facing many troubles with existing working features of iOS10. After updating my iPhone 6s plus to ios 11 user can't lower the volume on iphone properly. Trying to reduce the volume with the volume button but the sound is still at max which is annoying when sitting in some public place. To check and fix the issue make sure you try below listed options:
How to fix , not able to lower the volume of iphone using side button:
This may be the case where your device settings have messed up after upgrade to new iOS 11, so to fix the issue: Go to-> Sound settings -> turn on the button called "Change with Buttons". After this you should be able to control the volume of your iphone using the button as before.