Yes, your LG G5 is covered under the remainder of the one-year manufacturers' warranty.
Are you seeing any error messages on-screen when you press the volume buttons?
Do you hear an audible 'click' sound when pressing the volume buttons? Note that this sound is heard from the volume button itself, not from the phones' speaker. If no click sound is heard, the phone will have to be sent in for warranty repair.
If you hear an audible click from the volume buttons, try putting the LG G5 into Safe Mode to see if the volume buttons work in this mode. If the volume buttons work in safe mode, an app or setting may be preventing the volume buttons from functioning correctly.