Sounds settings changed on own and can't be reset
Try an hard-reset by pressing the Home and Lock buttons simultaneously in 10 seconds and then release them. Your phone will now restart and you will not lose any data.
Do an reset of all settings. To do this: Make sure you have a backup of your phone first or know the important settings for you. Then you go to the "Settings" application -> General -> Reset -> Reset All Settings. When you reset all settings, only the settings, no files will be affected.
Try reseting the whole phone and restoring it from an iCloud or iTunes back-up. To do this: Make sure you backed up everything to iCloud or your computer (don't forget the pictures). Then you make a reset from "Settings" application -> General -> Reset -> Erase All Content and Settings.


Did it work? Didn't it? If you have any further questions, please ask in the comments.