The configuration file /etc/AudioPara4.csv in the Android OS controls device-specific volume levels. The solution was to manually replace the configuration file with the original Vodafone AU version.
Download the file AudioPara4.csv from here.
Using the Android SDK push the file onto the phone:
$ adb remount
$ adb push AudioPara4.csv /etc/
627 KB/s (54136 bytes in 0.084s)
$ adb shell reboot