1 – first jailbreak
2 – For change the limit of volume, don’t use the app “volume boost” or other like this. The better solution, it’s change the file in system. Connect to iphone via SSH.
2.1 – Go to System/Library/PrivateFrameworks/Celestial.framework
2.2 – Take on your computer “ RegionalVolumeLimits.plist” and make a copie.
2.3 – Open the file with a soft like Plistedit pro (freeware)
there is a same soft on windows, but i have not tried it.
2.4 – Change all value : <real>0.82999999999999996</real> to <real>1</real> (write 1,00000000000000000)
2.5 – save and replace on iphone the file
2.6 – Restart iphone, push the volume and it should work.
Images and instructions courtesy of François, Thanks for the hard work.