I then connected the hdmi of stb to my denon 1910 avr and to my surprise I did not get dolby surround sound in any of the HD channels. I rebooted the STB , my avr and tried for sometime but to no avail.
Later in the night, I connected optical out from stb to my avr and then it was available !!
Again called the team lead today (I was sure CC would not be able to understand this ) and explained the issue. He conferenced me to some other guy and then after explaining the issue to both of them came to know that dolby is not available thru hdmi. This is as per design !!!

Can you beleive it ?
They just assumed that everyone would connect the hdmi to tv directly and hence its a waste to provide dolby thru hdmi !! I could not beleive my ears !!.
Both of them said that they will take my feedback and let their superiors know about this issue.
They refused to accept it as a defect
Right now I have connected stb hdmi to TV directly and the optical out to my avr.