Found a solution.

See also: community.home-assistant.io/t/…/793219

The following configuration works.

Note the go2rtc -> streams -> <name>
Is the same as the cameras -> <name>
They use the same IP address.
This is a Tapo C120 cam.

Note the “tapo://” protocol (this enabled 2-way audio) requires only the password, and device IP without a port number or URI path.

config.yaml

go2rtc:
  streams:
    living_room:
      - rtsp://username:password@xxx.xxx.xxx.123:554/stream1
      - tapo://password@xxx.xxx.xxx.123

cameras:

  living_room:

    enabled: true

    ffmpeg:
      inputs:
        - path: rtsp://username:password@xxx.xxx.xxx.123:554/stream1
          roles:
            - detect
...
...