You connect the cable box to your TV (probably with HDMI). You also connect the HTiB to the TV with HDMI. If your TV has an HDMI ARC (audio return channel) input use that one. Turn on ARC on the TV and the HTiB.
If your TV does not have ARC then you have to make an additional connection from the HTiB to the TV with an optical audio or analog audio RCA cable to get the sound from the TV to the HTiB.
If you TV has no audio outputs then you connect either of these same cables directly from the cable box to the HTiB, Check the cable box menu to see if you need to make a change in the audio menu. Turn the volume of the TV down when you want to not hear the TV internal speakers.