I have just finished upgrading my Xperia Z1 phone to the latest firmware update from Sony to fix the sound issue that was transfered to internal instead of external headset. This makes the sound so small and it's quite annoying if you are making a phone call as the sound become so hard to hear.
The new firmware has a build number: 14.3.A.0.757 and you will find it once you start your Sony PC Companion and connect your phone on it. Once you agreed with the update, it will start downloading the firmware, restarting your phone and start the upgrade process. After it has been completed, you can unplug your phone and turn on your phone and it will continue with the post-upgrade phase where it will try to optimizing your apps. That simple.
I do hope Sony also fixed the camera bug which caused the phone to consume lots of memory while using it.