Possibility that your TV does not support Dolby Digital. If so the STB must be changed to output PCM only. Settings on newer (VMS and IP) boxes is in the regular menu. For others you need to reach the hidden menus
- From on, power off just the STB. Usually best to do this at the STB as you want the TV to remain on.
- Hit OK on the remote
- Hit Menu on the remote
- Head to the HDMI menu and set L-PCM
Note this is all timing dependent so you may have to try a few times. Note if you hit OK in the third step a different hidden diagnostic menu shows up.
