A lot depends on how you have things connected. If you have the STB connected to your TV directly using HDMI and then an optical link to your Home Theatre System be aware that the majority of TVs DO NOT pass through DD 5.1, rather just 2 channel stereo.
To get DD 5.1 on a HTS you should connect the STB to an input on the HTS and then output from the HTS to the TV.
Some TVs do pass through DD 5.1 (or better) but they are in the vast minority at the moment. Most people using Soundbars only get 2 channel stereo as they normally connect from the TV to the soundbar via the optical out of the TV. A few of the more expensive soundbars have multiple HDMI inputs to allow them to take advantage of the DD sound.