This is something easily fixable on my end, but I think I've run into this issue or similar prior without knowing what had caused it. I am running the Steam version on Windows 10. 

If PNGtuber+  is running, and there are no output devices enabled in the windows Sound Control Panel, PNGtuber+ will no longer detect any Microphone input. The green bar indicating microphone volume will jitter up and down on a low volume instead.  

I discovered this because I had my speakers disabled, so that my laptop would only use my bluetooth headphones. Once reconnected PNGtuber+ worked as normal again (same when speakers enabled).

This also happens if a new output device is removed, added or used after PNGtuber+ has been opened, but it goes back to normal once Microphone input has been reselected. I also found this out with my Bluetooth headphones, which do have a built in mic, but it is disabled.