The first instalment of the TV adaptation of Daphne du Maurier's classic gothic novel, starring Downton Abbey actress Jessica Brown Findlay, was broadcast on BBC1 last night.
Viewers complained that they had to switch on the subtitles or turn up the volume to the maximum setting to understand what the cast were saying.
A BBC spokeswoman said that the corporation, which received 107 complaints, was reviewing the episode following "issues with the sound levels".
Comedian Al Murray was among those attempting to make out what was being said by the stars of the atmospheric drama.
"Find out what happens next in Jamaica Inn by getting your ears syringed!," he wrote on Twitter.
Choreographer and former Strictly Come Dancing judge Arlene Phillips wrote on the social networking site: "Did anyone else have trouble hearing BBC One Jamaica Inn."
John Challis, known for his role as Boycie in Only Fools And Horses, wrote: "Jamaica Inn LOOKS very good but I haven't heard a single word...Either the actors are mumbling or the sound track is faulty."
A viewer wrote: "Oh dear I know BBC1 is keen on authentic but the dialogue in Jamaica Inn is incomprehensible."
Another added: "Jamaica Inn starring Jessica Brown Findlay: Daphne du Maurier's story is a dark tale of sex and smuggling if you could hear it."
Another complained: "Jamaica Inn mumble mumble. Couldn't understand a word of inn keeper. Terrible."
Emma Frost, who penned the screenplay, suggested that a technical fault, rather than the way that the cast delivered the lines, was to blame.
"No surprises here - I'm told there was a major sound problem for tonight's broadcast of Jamaica Inn - not surprised you couldn't hear it," she wrote.
A BBC spokeswoman said that the drama was watched by 6.1 million viewers.
"There were issues with the sound levels last night that we are currently reviewing ahead of tonight's episode," she said.
The complaints come almost a year after 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.
The drama, set in 1821 against the windswept Cornish moors, was directed by Philippa Lowthorpe, whose credits include Call The Midwife.
The three-part series follows Mary Yellan who is forced to live with her aunt and domineering uncle following the death of her mother.
The BBC later apologised to viewers and said that the "sound levels" could not be altered while the drama was on air.
But it said that it had adjusted the dialogue levels for tonight's second episode.
The corporation said in a statement: "There were issues with the sound levels last night and for technical reasons they could not be altered during transmission.
"We are adjusting the dialogue levels in episode two and three to address audience concerns so they can enjoy the rest of the drama and would like to apologise to those viewers who were affected."
Writer Ms Frost wrote on Twitter: "It sounded like listening through mud (ironically ha). There was no problem on the version I saw but tonight I couldn't hear."
She added: "The director and execs were on the phone to the BBC from the off yelling 'why can't we hear it???' Too late I fear."
She wrote: "Complaints were relentless - quite rightly. None of (the) production team know what happened with the TX (transmission) sound. It was fine before."