Env: Oracle Linux Unbreakable 7 XFCE
It was running xfce4-mixer as old style audio adjusting way.
replacing is not a easy way even that pluseaudio was involved early as 2008.
cleanup all standalone installation which is xfce deps, such as im-chooser-xfce4.
then # yum install pluseaudio-plugin
which will erase xfce4-mixer as expected.