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That doesn't seem right as the notification sound is not the same as music/video/games/other media volume.
boot on safe mode and check again
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008
That's quite odd, what if you re-install the latest update with PCC
Bridge (for Mac)
Alternatives on How to backup Xperias
http://talk.sonymobile.com/thread/36355