I Just figured out why I couldn't hear the voices in the 6 channel version: the voices were in the center channel and if I go (in adobe audition) to Edit > Preferences > Audio Channel Mapping I can see that the Left and Right Channels are mapped to my speakers, but the center channel, LFE, LS, RS, et. al. are not mapped at all. Mapping the center channel to my left speaker allows me to hear the voices. So perhaps I shall revisit this project again one day and fix it in the 5.1 mix.

Did Eac3To get the channels right when it converted from the DTS-HD? Shouldn't the main sound be in the L & R channels, not the center channel? Is there a command line option that would correct this (if indeed it is incorrect)?