Never heard of a theme affecting volume, but I guess it is
possible. First try a battery pull and see if that helps.
If it does not uninstall the theme and test you volume.
If it reverts to normal you may have too give up that
theme.
I understand not wanting to, but to verify that it is
the theme causing the problem, you really are going
to have to. If the problem goes away, reinstall the theme.
If it returns the theme is indeed the culprit and you will
have 2 choices, lose it or live with the volume issue.