The adaptation of Daphne du Maurier's novel has attracted a total of 2,182 complaints about mumbled dialogue.
An average of 4.1 million people tuned in to the third and final episode. That was two million fewer than watched the opening instalment on Monday.
The BBC apologised on Tuesday for "issues with the sound levels".
The corporation's controller of drama commissioning, Ben Stephenson, later said the problem was partly down to the actors.
"Actors not being clear is obviously one part of it but my understanding about the complaints about Jamaica Inn was more complex than that," he said.
"I think it's probably not right to just single out that, but clearly we want actors to speak clearly. Of course we want them to give brilliant performances and you've got to respect that but if no-one can understand what they're saying then there is a problem."
After the first episode was aired, the drama's screenwriter Emma Frost said it was "like listening through mud" and took to Twitter to blame a technical fault.
In its statement on Tuesday, the BBC said sound levels on the second and third episodes would be adjusted before they were broadcast and apologised "to those viewers who were affected".
But on Thursday, a BBC spokesperson said 1,384 complaints had been received since Wednesday, taking the total to 2,182.
Jamaica Inn's problems follow similar complaints about dramas like Birdsong and Parade's End and a promise last year from the corporation's director general Tony Hall to address the issue of mumbling.
"I don't want to sound like a grumpy old man, but I also think muttering is something we could have a look at," Lord Hall said last July.
"Actors muttering can be testing - you find you have missed a line... you have to remember that you have an audience."
Complaints about sound problems on BBC costume drama Jamaica Inn have jumped to almost 2,200 after the final instalment was screened on Wednesday.