I have the same phone with the same issue. The drastic decrease in volume happened when I migrated to Android 4.4.2
the issue is pretty well documented in blogs across the internet.
the only way to fix is do downgrade back to 4.3 or 4.2, or upgrade to lollipop, or swap out the file in the phone that determines volume settings. ALL OF THESE SOLUTION REQUIRE YOU TO ROOT THE PHONE.
I have had my phone 6 months. Sprints response is I am basically S.O.L.
Corporate Sprint says take it to a service center.
The service center says it's not a hardware issue, it's an android (software) issue.
Bottom line is some programmer screwed up the volume setting, and it got released and is causing problems for some LG G2 phones.
Sprint shows no record of a planned fix. Sprint has told me there is no way to fix my phone and they will not replace it