Share your experience!
Some intensive phone operations might cause your phone temperature to rise, that's normal. But if you feel that it's over heating from simple operations, I'd suggest that you do a PC Companion phone repair, this will do a clean installation of the latest firmware and will also wipe your internal memory.
Open PC companion -> Support zone -> Phone/tablet software update -> Repair my phone/tablet (blue link style). Follow the steps given, do not connect your phone until prompted to press volume down while plugging USB cable.
http://www.sonymobile.com/gb/tools/pc-companion/
This is normal with long Camera usage, especially if you live in a hot weather. In my country now the temperature may rize to 45 degrees, and my camera shuts down for cooling after 2 pictures 
For the volume button, contact Sony support center to get it fixed:
/t5/FAQ/How-to-get-your-Sony-Xperia-product-repaired/m-p/991374#U991374
But in my country now not hot weather 30 degrees. I tested my other Sony St25i not any overheating plroblem in camera. When I write in this forum CPU heating is 50-55 degrees.Try PC Companion repair in my first post and report back.
You can test it in a Service center only, you don't have diagnostic tools for this as a user.
If you can't find it in the Contact US page, then contact your local distributor or merchant you baught the phone from.