Share your experience!
What if you boot on safe mode? do you experience the same issue?
How to boot on safe mode
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008
Done that. They do not have a clue and sent me to service. Probably I will get both phones there and see what comes...And very likely I will sell them both and switch to something different. As I can see around here Sony latest devices are full of problems and online customer support is bad.
My xperia z3c started to work by itself. With z3, same problem. Reading on forums, it seems it may be a problem with water. If you submerged it, or it encountered high temperature drops, I mean you used it outside at -10 C degrees and afyer that indoors at + 22C degrees, it may have some kind of protection and volume on speaker drops by itself. In this case you have to put it for few hours in a bowl with rice to allow the water to be absorbed. But again, that is what I found on different forums, especially z1 and ultra forums. It is not a z3 problem, it is an xperia problem. Anyway, keep it out of water, is is not in any way waterproof water-resistant or how you want to call it. otherwise a very nice phone