I am curious about this as well. I've noticed that I can't adjust the ring volume from the volume buttons on the side anymore either. I have to go into settings -> sounds to adjust the ring volume. It would be great if there is a fix for this. Otherwise I'm deleting volume button extender.
Same thing happend to me as well. I ended up deleting the volume button extender app because I tried many things and still got the same problems.
If you uninstall it you have to reboot the phone in order to regain normal control of the volume buttons.
Hope there's a fix to this soon because I really did use it. Since I play drums being able to go back and forth with the side buttons makes it alot easier.