Option 1 Use the headphone jack into an analog input of the HTS. This will sound the worse
Option 2 Split the audio output of the set top box with 2 Y single male to two female RCA splitters. Connect one pair of L/R to the TV and the other to the HTS.
Option 3 Connect the set top box digital audio output to the HTS. If the HTS has digital audio in you will get the best sound this way.