Ahhhhhh.. They aren't "hidden" at all. The actual sounds are compressed and stored in the ".fsb" files. (I had a detail wrong above. The ".erf" files contain the triggers and the mappings, not the voice-over recordings.)
The ".fsb" file are located in: "\<game install folder>\modules\single player\audio\vo\<language>\vo"
The "alistair_main.fsb" is over 52MB, and the largest is the "party_ran_banter.fsb" at over 76MB!
See this discussion on the BSN: Any way of getting the audio source files?
But note that they raised the same issue I did, it's all snippets. No "conversations" you can just play through.