The BBC has promised to "look at" sound levels for its new drama SS-GB after viewers complained about mumbling.
Set in a fictional London under Nazi occupation, the first episode aired on Sunday and got 6.1m viewers.
Around 100 complaints had come into the BBC 24 hours after it went out, with numbers rising on Tuesday.
The BBC said: "We take audibility seriously and we will look at the sound levels on the programme in time for the next episode."
The drama stars Sam Riley as an officer working simultaneously with the Nazis and the British Resistance.
Some viewers took to Twitter to air their views on the sound levels.
One user Luanne wrote: "I am 12 minutes into SS-GB & I'm turning on the subtitles. All this tough guy, breathy, growling, mumbling, I can't understand you!"
TV critic Emma Bullimore told the BBC: "We are living in astonishingly ambitious times for TV drama, attracting world class talent to the small screen, and sometimes this means the basics of storytelling get slightly overlooked - either in terms of audibility or making a story too complicated or difficult to follow (as Sherlock was criticised for).
"However, it does feel that mumbling is the latest bandwagon for BBC bashers to jump on and the odd muffled word gets spun into a story. Actually, my enjoyment of SS-GB wasn't marred by sound issues."
It's not the first time the issue of mumbling on TV dramas has been raised, with past programmes such as Jamaica Inn and Happy Valley having hit the headlines over the audio.
Back in 2013, the BBC's director general, Tony Hall, told the Radio Times: "I don't want to sound like a grumpy old man but I think muttering is something we could look at."
Analysis - do flat TVs mean flat sound? Leo Kelion, technology desk editor
As televisions get thinner, their makers have less space to build in speakers unless they opt to add sound equipment to the sides and/or below the screens, threatening to make them look less "sleek".
In many cases families might be best advised to buy a separate soundbar or speaker system. This isn't just the case for budget TVs. In many cases manufacturers assume those that can afford the higher-end models will invest in the extra technology.
Sony recently took a new approach to the problem with a TV that has no dedicated speakers but rather produces sound by vibrating the OLED screen itself. The jury is still out on how well that works.
But in general, if the vast majority of programmes on yours set sound fine and only a few don't, then the problem is likely to be with how they have been mixed rather than the TVs themselves.
Speaking on the Today programme, ITV's Victoria creator, Daisy Goodwin, pointed out that Riley may have made a conscious choice to not speak with perfectly clear diction in keeping with his character.
SS-GB, a five-part drama, also stars Kate Bosworth as a US journalist for the New York Times.