Audio coming from certain content providers or devices (such as Netflix, Amazon Prime Videos, Roku Streaming device, Apple TV, etc.) typically use an advanced digital audio signal such as HDMI or advanced Dolby audio. These cannot be translated into analog or PCM (pulse-code modulation) by most television software, this means the signal will not passed through into the headphones. As a result, you may get silence or audio from a different source like your cable box.
In order to resolve this, you will need to change the audio output format in the app or device to PCM (may be in the smart hub on some TVs) or you will need a digital to analog decoder. The Digital to Analog Decoder will take the advanced digital signal and decode it into an analog signal that your headphones can utilize.