but thinking about 7600 on the transponder MIGHT, key word MIGHT, have made the legacy pilots look at the transponder and see if there was a reply light illuminating at regular intervals.
if you have charts for brazil, does the airway/jet route show "communications gap" for the area involved?
also, did the legacy pilots attempt contact on all comm radios? some comm radios are wired to the top antenna, some to the bottom and sometimes (not always) this might make a difference.