[Comment from a bug triager]
That's because the Volume control is not a plasmoid (like the Device Notifier, Battery), but a system tray icon, which only renders a standard Qt popup.

Adding dependency on wish request about making the KMix plasmoid.

Removing dependency, as I was able to fix it in the native tray w/o the help of the Plasmoid.

Fixed in KDE 4.13

(In reply to comment #4)
> Fixed in KDE 4.13

I should note that I tested it with the panel on all 4 sides: Top, Left, Bottom and Right

Git commit 0f7a603e1d327f76dbd6a62d08315b1956794d42 by Christian Esken.
Committed on 20/12/2013 at 10:17.
Pushed by esken into branch 'master'.

Configuration menu is now using Tabs to provide a more standard,
  convenient and pleasing layout
 Sound Menu: Play button reflects playback status and shows either
  "Play" or "Pause"
 Bugfixes and Features (CCBUGS, as most bugs are already closed). Hint: Some
bugs might have actually been alreay fixed ibn KDE4.11 or KDE4.12.

Bug ID	Severity	Summary
Related: bug 317926, bug 20, bug 315383, bug 299477, bug 304144, bug 296951, bug 303608, bug 214854, bug 319600
applet
volume control
position.

M  +9    -5    CMakeLists.txt
M  +41   -20   ChangeLog
M  +5    -1    apps/KMixApp.cpp
M  +167  -238  apps/kmix.cpp
M  +8    -13   apps/kmix.h
M  +3    -0    apps/kmixctrl.cpp
M  +3    -0    apps/kmixd.cpp
M  +18   -12   backends/mixer_backend.cpp
M  +1    -0    backends/mixer_backend.h
M  +229  -150  backends/mixer_mpris2.cpp
M  +23   -6    backends/mixer_mpris2.h
M  +8    -2    backends/mixer_oss.cpp
M  +1    -1    backends/mixer_pulse.cpp
M  +9    -9    core/ControlManager.cpp
M  +102  -15   core/GlobalConfig.cpp
M  +105  -47   core/GlobalConfig.h
M  +65   -31   core/kmixdevicemanager.cpp
M  +9    -4    core/mixdevice.cpp
M  +4    -12   core/mixdevice.h
M  +6    -4    core/mixer.cpp
M  +16   -10   core/mixertoolbox.cpp
M  +1    -1    core/version.h
M  +17   -3    core/volume.cpp
M  +9    -8    core/volume.h
M  +19   -6    gui/dialogchoosebackends.cpp
M  +1    -1    gui/dialogchoosebackends.h
M  +9    -2    gui/dialogviewconfiguration.cpp
M  +37   -10   gui/guiprofile.cpp
M  +27   -24   gui/guiprofile.h
M  +93   -118  gui/kmixdockwidget.cpp
M  +3    -4    gui/kmixdockwidget.h
M  +13   -11   gui/kmixerwidget.cpp
M  +296  -91   gui/kmixprefdlg.cpp
M  +82   -32   gui/kmixprefdlg.h
M  +87   -38   gui/mdwslider.cpp
M  +4    -2    gui/mdwslider.h
M  +69   -42   gui/viewbase.cpp
M  +1    -0    gui/viewbase.h
M  +25   -30   gui/viewdockareapopup.cpp
M  +1    -1    gui/viewdockareapopup.h
M  +4    -4    gui/viewsliders.cpp
M  +1    -1    gui/volumesliderextradata.h

http://commits.kde.org/kmix/0f7a603e1d327f76dbd6a62d08315b1956794d42