Viewers complained about Jamaica Inn last month despite the corporation saying it adjusted sound levels "to address audience concerns" after viewers said they were unable to hear some of the dialogue.
Many said they had to turn up the volume or put subtitles on for the TV adaptation of Daphne du Maurier's classic gothic novel.
Last year BBC director-general Tony Hall said the corporation could look at how to stop actors "muttering" in its TV dramas.
"I don't want to sound like a grumpy old man, but I also think muttering is something we could have a look at," he said. "Actors muttering can be testing - you find you have missed a line... you have to remember that you have an audience."
He said that the corporation was addressing the problem of background music making it difficult for some, particularly older viewers, to hear what was being said.
