You make it sound like this is some rampant problem. I have played both DAO and DA2 numerous times and am on the third play of DAI. I also play with full sound and subtitles on so I read along while the NPCs are talking. Problems like this are extremely rare in BioWare games and generally the voice direction is fantastic.
There are hundreds of hours of voice work in each game. If a few lines here and there are less than adequate I think that is an acceptable error ratio.
The most egregious mistake I've noticed across all three games was a line in DAO. You can ask the dwarven proving master to explain the provings and he says that the combatants enter a ring of "scared [afraid] stone" instead of what is written in the subtitle, the correct "sacred [holy] stone".
The next common mistake I've seen a few times throughout all three games is the emphasis placed on the incorrect word in the sentence, which is even more obvious when it is italicized in the subtitles. One of the more amusing examples of this is in DA2 because the difference here is between male and female Hawke. When talking to Aveline to start the quest The Way It Should be, fem Hawke says, "Alright Aveline, you have something worth doing?" While male Hawke says, "Alright Aveline, you have something worth doing?" In this instance I think the male version sounds better, but there is not an emphasis in the subtitle, so I don't know the exact intent by the writer.