I have a G4 and this doesn't happen to me. When you use the volume controls for up / down, I presume the volume indication increases or decreases.
I guess the first thing a VZW rep would advise is try booting the phone into Safe Mode. By doing so, you are only running the apps that are provided by VZW (i.e. not any that you downloaded). Did the symptom go away when making a call? If so, maybe there's a rogue application that's responsible, or maybe your most recent OS update was left a bit dirty... The latter specially if this behavior started after doing an Android update.
If the symptom did go away, I guess I'd perform what's called a Factory Data Reset, after I backed up all my important stuff and settings. In this way, I could rule out an Android Update not finishing "cleanly". You see, the updates are programmed and tested under the assumption that a phone has no downloads on it and that someone did a FDR prior to the install. We often get "lucky" when we do a system update to Android but don't do a Factory Data Reset and things all work fine!
In many years of using Android phones, I think I'm perhaps 8 out of 10 times lucky. Twice after updates, I had to do an FDR to restore proper operation of the device.
Then I'd start to add back in downloaded apps, one at a time and I'd give it some time between downloads to make sure I had a good chance of identifying which app is responsible if the symptom returns. Likely the FDR was necessary and you won't find a rogue app.
Let's say you are hesitant to doing a FDR. Understandable. Do you have any Music Player, Equalizer, Video/Movie Player, Browser, or News / Newspaper or similar reading program or any other Audio streaming type program that you downloaded? I'd try uninstalling them first to see if I could correct the behavior without an FDR.
It could be hardware related, but if it's only adjusting after a call, or on power-up, it really seems like software.