Low volume depends on many settings. Often it's a user problem with wrong settings. Changing the region may help, because in some countries (e.g. EU) maximum volume is limited by laws. Using an audio player like PowerAmp that supports pre-gain setting and HD hardware audio also helps.
Fiddling arround with system files on rooted phones mostly helps, but it's hard to find infomation about. Take a look at /system/vendor/etc/mixer_path***.xml. These files define sound properties. Find the right file and find the right setting but be careful to not mess up your device.