Note that it has two enum variables, useAudio_ and useClock_. It was to debug the program without audio transcoding and clock overlay. *PS: recoding audio in public places is illegal in most of states; plz compile it with true at your own risk.
The clock overlay has a property, "time-format". It takes same format as the function, std::strftime(), takes.
For more debugging output, you can set an environment variable: export GST_DEBUG=*:3
One thing I noticed is that the video frame rate is very low. I am not sure if it is due to the H.264 encoder performance or MPEG2 decoder performance. It got better when I assigned more memory, 256MB, to GPU but it is still not good enough.
I ordered a MPEG2 decoder license from Raspberry Pi Store: $3.28 USD. They said the license code will be email within 72 hours. Once I get the license, I will re-test the performance.