I'm a sound engineer, I've worked on film, TV & radio & in particular dubbing theatres where they mix the final soundtrack for movies & big TV dramas. There are three main problems:
- Voice production, ie actors mumbling their lines. This is worst in US films because North American accents are harder to understand, they have a lot of elision, ie sounds are missed out & blended into one another. Say an actor has to say the line "I know a little guy who can do that for you." In typical US English, that whole sentence can be spoken with hardly any audible consonants, which are generally the parts of speech that contain high frequencies and transients (sounds with distinct beginnings). You might get a hint of transient or two at the beginning of 'can' and 'do', maybe a fricative f on 'for' but otherwise it's a difficult-to-distinguish string of vowel sounds - ie a drawl. Say the same line in British English - or African or Australian or West Indian - and the consonants are back, chopping up the vowels and giving your ear & brain lots of clues as to what words are being said.
- Poor sound mixing, especially for TV. Proper movies are meant to be seen on a big screen, with a correspondingly big soundtrack. The loudest sounds in a typical movie soundtrack are music and sound effects. Dialogue has a lower average level, but this is usually fine, when listened to at typical cinema sound volumes, ie quite loud. You get exciting bang-crash-music action scenes alongside dialogue at a more comfortable volume. If the dialogue was mixed to the same loudness as the music/SFX, it would be unpleasantly boomy.
But for TV, average listening volumes are lower, yes even if you have a 'home cinema system'. You're in a much smaller room, and most of us have neighbours/children to worry about, plus a certain amount of envirinmental background noise - traffic/kitchen/plumbing/teenagers etc. So dialogue levels must be RAISED to roughly equal the music/SFX. This has generally been known in the industry as 'mixing for the small screen'. Living rooms, even with 52-inch plasma tellies, are not cinemas.
But professional TV sound mixers work in very high tech, quiet sound studios, and the art of mixing for the small screen is being lost. Many TV dramas now resemble small movies in budget and general 'production values', and sound mixers succumb to the temptation to mix their soundtracks at high listening volumes, and make a big, fat movie soundtrack. The result, when reproduced at typical living room volumes, through typical small TV speakers, is dialogue that often borders on inaudibility.
- No-one remembers to "be the listener". Audiences only get one chance to understand the dialogue. During post-production, scenes are played back hundreds, thousands of times. And everyone's got a script, so we know what the actors are supposed to be saying. Once you know that, it's practically impossible to make your brain forget it. Sound mixers should act as the listener, the first hearer. If it's not understandable at first hearing, then that represents failure of the soundtrack. I've had producers turn to me during mixing sessions when I've pointed out inaudible dialogue, and point out the lines on the script. Or they simply tell you what the line is. And I say, yes I know YOU know what he's saying but that's because you've heard it a thousand times, with the script in your brain. The audience don't have that. The irony being, of course, that now we do, after a fashion, and we are rewinding scenes à nd switching the subtitles on!
There are complications to all this, of course: the mix of music/SFX
under dialogue is also a big issue, as is the question of how stereo and 'surround' tracks sound when heard in mono. But these are the three biggies, and the audiovisual industry generally needs to sort them out.