On stock Android it's in sound settings (generic, not in the app). Is it similar on the G2? Settings > Sounds > Audio Effects
take a look at the first post on this thread
http://forums.androidcentral.com/verizon-lg-g2/316707-rear-volume-buttons-alternative.html
It references a overlay app of sorts that allows you adjust the volume at anytime. You can also set it up t show the equalizer.
This app adds an eq to any sound on your phone...good not great. Better than nothing on play music.
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