Further info for future Xperia phones you may buy: when you unlock your bootloader your DRM keys get wiped, without them, OTA updates, Mobile Bravia Engine (in models that have them -- not the case for Xperia M), Track ID, Download Music Info and SensME (alongside some other proprietary apps/features) will not work, even if you flash stock firmware again.
Before unlocking your bootloader it's always recommended to do a TA backup first, so, if you need, you can relock your bootloader to get your DRM keys back and make proprietary Sony apps/features working again. Since you didn't backup your TA partition before unlocking, you are out of luck now and there is no way of relocking your bootloader.
Also, DO NOT EVER TRY restoring a TA backup from another device (even if it's exactly the same model), TA partition contain some unique info and flashing a TA backup from another device will permanently hard brick your phone...
Sony Xperia M dual [C2004]
ROM: 15.5.A.1.5 (stock)
Android version: 4.3 (rooted)
Bootloader: locked (unlockable)