I just noticed that after the upgrade to plasma 5.4 (and probably a reboot) now I have two volume icons in the system tray: the new one is coming from plasma5-pa while the old one, according to this  archlinux forum entry, from kmix.
According to the package spec file , plasma5-pa is the replacement of kmix. Shouldn't plasma5-pa either obsolete kmix or somehow prevent it from starting at login? For the case obsoleting kmix is correct I've attached a modified spec and changes files for the plasma5-pa package with "Obsoletes: kmix" added .
In the meanwhile I might try to remove kmix by hand.
 https://bbs.archlinux.org/viewtopic.php?id=201805  https://build.opensuse.org/package/view_file/openSUSE:Factory/plasma5-pa/pla...  I know that there is some kind of gist in the opensuse servers, but I cannot find it