On a crowded commuter train the morning after SS-GB was transmitted, I noted that more than half of everything anyone said – either person to person or on a phone – had to be repeated at least once. The reasons for these failures to hear will include: not listening, obduracy, background distraction, attempts at discretion, or a decline in the auditory canals.

Yet, when those same ears are listening to a TV drama, they are expected to catch first-time lines likely to be more data-packed, enigmatic or epigrammatic than everyday conversation. So it is unsurprising that one of the sounds most often heard from TV viewers these days is complaints that they haven’t heard the sound.

There are two strongly disputing viewpoints on this issue. Baffled listeners contend that modern actors – no longer trained to deliver a line in London so that it bounces off a rock at Land’s End – mumble on screen, and then receive insufficient amplification from technicians. A more tolerant explanation is that many TV writers and performers are aiming to reproduce as precisely as possible how people really speak, which may sometimes necessitate a greater listening effort from the audience.

Until SS-GB, the BBC dramas that had faced most accusations of inaudibility were Jamaica Inn, Taboo and Happy Valley. But the central characters in SS-GB are living under Nazi occupation, while Jamaica Inn is about smugglers, and Taboo involves thieves. For such people, being overheard might mean disaster and so conversation is often soft, swallowed or with shadows swathing lips. In those cases, reduced audibility may be realism. And Happy Valley is mainly spoken in Yorkshire dialect and so – before ordering mass enunciation lessons – it would be useful to know if that show mainly defeated ears that were tuned to RP (received pronunciation).

The BBC has pledged to “look at” (listening to might be more useful) the sound levels before episode two of SS-GB on Sunday. However, any national audibility level for TV drama is a problematic concept. Guidelines are difficult to set because the general population will contain such wide ranges of concentration, ear wax and adaptability to dialect or slang. And if TV actors were required to declaim every line for maximum audibility and clarity – as seems to have been the practice, for example, in Victorian theatre – there would be a significant loss of realism.

Humans speak at different levels depending on whether they are delivering a lecture, making love, confiding a secret, or expressing a view that they hardly dare share. We speak slower in grief, quicker in love.

So, if drama wants to be convincing, TV characters must be allowed mouths that mutter as well as shout. Viewers who consistently have trouble comprehending dialogue across a range of shows might be gently advised to accept that they have some degree of hearing impairment or attention deficit, and reach for the volume switch or subtitles which, after all, are provided for this purpose.