1. Go into the NowTV box settings and play around with the Audio settings (start with stereo).
2. When watching something on your NowTV Pass press the star * button on your NowTV remote where a Volume mode option will appear on screen (toggle between the different modes) and see if the off mode makes any difference.
3. With the power switched off to both your television and box temporary disconnect the HDMI cable at both ends.
4. Try another HDMI port on your television and another HDMI cable between the television and NowTV box.
5. Test the box on a different TV if you have one.