The channel count comes from IAudioClient::GetMixFormat()
Help and support for HDHomeRun DVR and HDHomeRun software for Windows 10, Mac, Android, XBox, etc.
Found the problem with Windows and fixed it.
The machine has two HDMI outputs, with one to the 4K TV and the other to (identified as) a "720p Generic Monitor" video-to-AV converter that does not remain connected when not in use. When it was connected AND the app was running, I discovered that Windows had locked the sound output of one of the app profiles (there were two profiles listed) to the AV converter.
I remember changing the system sound output (via the taskbar icon), not the app output. I was trying to get Windows to launch the app to the converter so perhaps Windows was trying to be "helpful" even though it always launched the app on the main display. BTW when I checked the app sound settings profile again after disconnecting the AV converter, there was only one profile listed.