The BBC's new period drama, Jamaica Inn, lost more than 1.5 million viewers – a quarter of its audience – for its second episode, after viewers who tuned into the launch on Monday were left frustrated after struggling to make out the dialogue.

The first instalment of the TV adaptation of Daphne du Maurier's classic gothic novel, starring Downton Abbey actress Jessica Brown Findlay saw viewers complain that they had to switch on the subtitles or turn up the volume to the maximum setting to understand what the cast were saying.

The problem was blamed on a technical fault.

A BBC spokeswoman said that the corporation, which received 107 complaints, was reviewing the episode following "issues with the sound levels".

The second episode was watched by 4.5 million viewers, down on over 6.1 million viewers.

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.

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."