If the movie or TV show you're watching has 5.1 available, make sure that option is selected within the Audio & Subtitles menu during playback.

Please note that not every episode or season of a 5.1 supported TV show will have 5.1 available. You can test for 5.1 availability of later seasons of a TV show by using the season drop-down option on the title description page. Additionally, not every TV show or movie supports 5.1 in every language.