Welcome to the community @spike 
I encountered something similar to this just a few days ago. When launching DaVinci Resolve Studio, my Plasma volume control would indicate a device appearing/disappearing rapidly, so long as Resolve was open, and no sound could be heard within Resolve.
For me, what it ended up being was simply needing to enable an audio recording device in KDE Plasma. The issue would occur if I disabled my microphone input (“Off”).
So instead of “Off”, I set it to:
