Hi,
Please refer to
$ gst-launch-1.0 nvarguscamerasrc num-buffers=300 ! nvv4l2h264enc ! h264parse ! queue ! mux. audiotestsrc num-buffers=1000 wave=1 ! voaacenc ! queue ! mux. audiotestsrc num-buffers=1000 ! voaacenc ! queue ! qtmux name=mux ! filesink location=a.mp4
You will see two audio tracks in a.mp4