Can't Adjust Phone Volume When Screen is Locked
Volume buttons on the phone and on the earbuds work fine for calls unless I am listening to music (or a podcast, etc), and the screen is locked.
If there is music playing while the screen is locked and a call comes in, I answer the call by clicking the earbud controller and I can't adjust the volume of the call. The volume buttons on the phone and on the earbuds do nothing. After the call is over and the music resumes, the effect of the volume changes kick in and the music is now really loud or soft, depending on how I was trying to change the volume during the call.
I've tried multiple sets of earbuds. I've tried cleaning out the jack port. Under Settings->Sounds, Change with buttons is enabled.
iPhone 5s, iOS 8.3