The Oxygen-Sys-App-Negative.ogg is installed as the "AC adapter unplugged" notification sound. If I reboot a system and unplug the adapter for the first time, the sound is played OK, but for further times it is played distorted. As if it was cut into several noncontiguous chunks and they are played quickly one after another. If I press the "Test sound" button on the configuration dialog, it is played fine. Sometimes it "fixes" itself, but it happens rarely.
If I set other sound as the notification, all works fine.

Some more details about your distribution and Plasma version as well as the hardware involved could be helpful here

ArchLinux, Plasma 5.18.3, Qt 5.14.1, Linux 5.5.10.arch1 on the hp ProBook 450 G3.
By the way, this issue has been identified long time ago, so I am not sure if this depends on the current software versions.

I have the same Plasma version and can't reproduce this here at all, could this be hardware related?