I am trying to get dolby 5.1 surround sound from fios arris vms1100 box to my AVR. The audio drops while I am watching a cable channel like ESPN but not while watching basic channels like ABC or CBS. The audio output on the BUI menu is set to AUTO. This happens whether I run an optical cable from the VMS1100 box to my AVR, extract the audio from the HDMI output to an optical cable or use HDMI ARC thru my TV set. Any advice would be appreciated.