there is nothing called HDMI ARC Cable.. Only ARC Ports..!
In TV's usually HDMI Port 1 acts as both HDMI & ARC Port..
As sumit bro told earlier u can't get Dolby effect from TV Speaker's.
TV's usually have 2 side speaker's or 1 main speaker and sub-woofers. This will not produce DD or DD+ Sound. These can produce only 2.0 OR 2.1 sound. You need an external source, at least a 5.1 speaker to produce DD Sound.
Your DD sound option is active in TV because u hav connected the HDMI cable from STB, to ARC port. There will be ZERO effect for this.
Just unplug the cable from HDMI1 and plug it to HDMI2, u can see the Dolby Digital option grayed out.
In addition to @Jithin91 Bro post
ARC is supposed to let you have one connection between your TV and whatever you use to create sound: a receiver or a sound bar . You send video to the TV with an HDMI cable, and that same HDMI cable sends audio from the TV back down the same cable. In the pic consider build in tuner as stb which is connected to tv.
After connecting and configuring home theater to tv with ARC ... make sure to place speakers in correct position.. this is very important.. then watch any movie with dolby support ... you'll definitely enjoy and you'll feel the audio is moving...you'll get surround sound like a movie theater.
Connectin LED TV through HDMI ARC connection to the Audio System is not enough,you have to make sure that HDMI Control is ON on your Receiver as well as TV..