Comment # 1 on bug 1091191 from Wolfgang Bauer

This *is* an upstream issue IMHO.

The code in Plasma passes an invalid QUrl to phonon IIRC.
(which happens to work with the VLC backend for some reason, but not gstreamer
which is the default)

Add "file://" in front of the file path in .config/plasma_notifyrc and it
should work, i.e. something like this:
[Event/startkde]
Action=Sound
Execute=
KTTS=
Logfile=
Sound=file:///home/test/Desktop/login.ogg   <=====
TTS=

Or install phonon4qt5-backend-vlc.

Closing as UPSTREAM for now, althoug I do intend to propose a fix myself.