All righty - these are the steps I took to root my ZX2 and tweak the system config to disable the volume cap. I don't guarantee this will work for you, and do it at your own risk - if you brick or damage your ZX2 it's not my fault! and the Chinese root tool will likely install some bloatware apps or who knows what other dodgy stuff it does
First select: Settings >> Security >> Unknown sources on your device.
Download the following apps from google play:
ES File Explorer (or other file explorer)
On your ZX2 download the APK from this URL: root.360.cn (mobile version download)
Go to your downloads folder and install the downloaded APK - accept security warnings. Open the rooting app and press the large center circular button (need to be connected to wifi).. the app will start trying to root your devices and display percentage - you might see security warnings from Android, just accept them and keep going - once it's done you might see a sad face, if so, try again by pressing the circular button.. eventually I managed to get successfull root (different icon to sad face), you're in business.. I had to attempt few times before it worked for me.
Once Rooted:
0. set root access in settings of ES File Explorer - root tool popup warning will come up > accept on left button i think (in Chinese)
1. you can access /configs/region.zip
2. copy paste region.zip back it up to your own folder just in case
3. unzip /configs/region.zip (you might need unzip app from playstore)
4. edit region.prop (you might need a text editor from play store):
ro.sony.volume_limit=0
ro.sony.walkman.euvollimit (add one more zero to the end of the integer value)
5. save it, zip it up and put it back like it was before /configs/region.zip but with the changed values
6. Restart your device and voila, louder by about 20-30%
7. Optional: uninstall root tool and bloatware