On Apple TV, Sky will default to streaming content in 5.1 audio, but it's possible that your Apple TV is still set to "stereo." In such a case, please check your Apple TV's audio settings: refer to Apple Support for guidance.
On other SmartTV platforms, make sure your audio settings are correctly configured for 5.1 audio.
Lastly, ensure that your audio system itself is properly set up for 5.1 audio playback.
If you continue to experience audio problems when using 5.1 audio, it is recommended to switch back to stereo audio for a seamless viewing experience.