Several ideas.
What is selected under
(rightclick >) Options > Preferences > Audio > Output?
Does selecting another output make a difference?
What is your OS? (re. Windows audio tapers)
Did you perhaps redefine keys under:
(rightclick >) Options > Preferences > Key definitions & WinLIRC?
To reset: select "Volume down" resp. "Volume up" and then click on (Reset to default)
[You may have to do this separately for windowed mode and full screen mode.]