I've googled this problem and seen that loads of other people seem to have it too - across many different phones. The device in question is an Xperia J. After choosing both a ringtone and notification alert from the ones already in the phone (ie, not custom MP3s), it works and plays both the selected files. But once the phone has been powered off and restarted, it switches both back to the defaults. The phone has never been connected to a computer, only powered off when not in use and charged via the mains cable. Can anyone help? Thanks in advance if you can!