Nothing is arriving in the transcript

When the system-audio permission is missing, macOS does not fail loudly. The tap is created successfully — right sample rate, no error — and then delivers frames in which every sample is zero. From the outside that is indistinguishable from a quiet room, which is why the symptom is silence rather than a message.

If you could hear the other side and nothing arrived: System Settings › Privacy & Security › Screen & System Audio Recording, tick MoPilot, then quit the app fully and open it again. The grant is only picked up at launch.