WASAPI is documented as not sending audio data if nothing is playing any sound, e.g. this note about WASAPI in NAudio:
Now there is one gotcha with WasapiLoopbackCapture. If no audio is playing whatsoever, then the DataAvailable event won't fire. So if you want to record "silence", one simple trick is to simply use an NAudio playback device to play silence through that device for the duration of time you're recording. Alternatively, you could insert silence yourself when you detect gaps in the incoming audio.
I have written a WASAPI listener, but on my machine, it always fires even when nothing is making any sound. Other users indeed report that when nothing is playing, the signal stops.
So I think some process must be making Windows think that something is playing on my machine, but I can't figure out what it is.
How can I find the source of this phantom audio?
This isn't much of an answer, but when I rebooted, the issue went away. I am assuming that some application like Spotify was playing something, even though it wasn't audible.