So regarding Pixel Streaming and how it handles audio, it only listens to the main audio device, e.g. GEngine->GetMainAudioDevice() (found in Engine\Plugins\Media\PixelStreaming\Source\PixelStreaming\Private\AudioSubmixCapturer.cpp)
If your third party plugin is not piping its audio into the main audio device, Pixel Streaming won’t capture it. Unfortunately there isn’t any way around this limitation at this time.