System sound mutes when "po" key sequence entered quickly - why!?

I have been bugged for months by seemingly random muting of the sound on my PC, making listening to music almost impossible. I think it may have started happening when I upgraded to Windows 10 but not sure. I have Kaspersky Internet Security and it hasn't detected any malware or virus, and I have checked all installed software and don't see anything suspicious.

I have narrowed down the issue to certain key sequences that I type, so far I have found that quick-typing "po" or "pu" seem to toggle the system mute - the widget below appears in the top left corner of my screen. If I reverse the characters it unmutes. This happens no matter what application I type it in (specifically tried Microsoft OneNote and both Edge and Chrome browsers) If I type the characters with a pause in between, the mute toggle does not happen.

Does anyone know what might be causing this and how to disable it? It seems like some kind of hotkey mapping. Any help would be much appreciated!!