These are the factors I have identified, in no particular order:
- Mumbling and/or whispering
- Speaking very rapidly
- Highly emotional speech (e.g. when crying, shouting or ranting)
- Use of unfamiliar words, including novel or regional slang, professional jargon and abbreviations
- Using dialect unfamiliar to the listener (encompasses unfamiliar vocabulary, grammar, accent and culture-specific referents)
- Non-standard or grammatically incoherent constructions
- Speaking in an accent that is unfamiliar to the listener
- Mispronunciations
- Monotonous delivery: little variation in volume, stress or emphasis
- Song lyrics are particularly hard to decipher, partly because they often make very little sense to begin with
- Uninteresting, excessively complicated or excessively specialized topic of discussion: lack of engagement or subject-matter comprehension encourages the listener's attention to wander, leading to misunderstanding of subsequent utterances (with associated sense of frustration)
- Unfamiliar subject matter, especially popular-culture references. When such references are not understood, the wider significance or general context of the utterance that contains it may be lost. Typical examples are references to decades-old TV shows and celebrities, actors and singers
- Ambiguous statements or puns, especially if the references are obscure
- Speaker's lips are not visible
- Speaker's facial expressions and/or body language do not match the informational or emotional content of their utterances.
- Conversation contains excessive non sequiturs
- Poor quality of recorded/transmitted sound (mobile phones/cellphones are abysmal for this -- mostly affects live broadcasts)
- Clipping of frequency range (again, mobile phones are a terrible offender -- mostly affects live broadcasts)
- Sound cutting in and out (e.g. due to poor phone reception or satellite link -- mostly affects live broadcasts)
- Masking of speech by excessively loud music and/or other background noise
- Other sensory overload: fast panning or cutting between short scenes, too much concurrent physical on-screen action, background noise, accompanying music, emotional content and rapid speech overwhelm the viewer's ability to decide what information is most relevant
- Excessive difference in speech volume between speakers: listeners find it hard to adjust their expectations of what they are about to hear quickly enough
- Except when the listener is conversing with someone in a live setting, no possibility of asking for clarification or repetition of what was said
- Deafness of the listener, including difficulty hearing particular frequency ranges
- Age-related slowing of signal processing: as most people get older, the rate at which they can interpret what they are hearing decreases, which means that rapid speech is harder for them to keep up with, especially if it is consistently rapid
- Incomplete, inaccurate, unavailable, poorly contrasting or badly synchronized subtitles. ('Inaccurate' includes the nasty habit some subtitlers have of substantially rewording sentences. And there ought to be a particularly unpleasant punishment for those decision-makers responsible for plastering subtitles right across the faces of the individuals speaking the lines to which they relate.)
Do other people also think that indecipherability of film and TV dialogue is a significant problem?