Where do the pronunciation variations come from?
First and foremost, let’s remember that improving pronunciation means trying to get closer to a standard that we perceive as being “the right one”, because it’s more widespread or more prestigious.
It is therefore more accurate to speak of a deviation from a desired standard than of a fault, since other ways of pronouncing are just as legitimate.
Except in pathological cases, language learning makes us all expert speakers of our mother tongue. It’s this ultra-specialization that, from a certain age onwards, leads to difficulties in perceiving and reproducing foreign sounds. Our brains have created the mental categories needed to recognize the sounds of our mother tongue, but not yet those needed to recognize new sounds. This deafness hampers our ability to understand the words we hear, and often prevents us from pronouncing them properly.
This is why the French speaker will typically have difficulty pronouncing the th which he will pronounce with a z (“zi book“instead ofthe book“). The American speaker, on the other hand, will have difficulty articulating the r à la française de “ragoûtThe Japanese will have great difficulty distinguishing between “round” and “round”.round“andlong“…
Making yourself understood, but also heard!
Whether we like it or not, and no matter how patient we are, we all have a tolerance threshold for foreign accents. If the sounds we hear do not respect the rhythm, accentuation, intonation and sonorities of our mother tongue, the compensatory work the brain does to process the message will trigger an automatic loss of attention and interest in the conversation.
Good pronunciation is essential for smooth, efficient and pleasant communication.
And unlike the lexicon, where there’s no limit to the number of words you can learn (you can always learn new words), the French language comprises around thirty sounds and a few fundamental intonations. It’s not unreasonable to think that mastering them is a very attainable goal!
How does the coaching work?
He will first clearly pronounce a sentence selected in advance, or choose one from the course. He will then repeat it in order to diagnose the type(s) of error(s).
Depending on the diagnosis, he will then reproduce the same sentence in a certain way, consciously playing with the sound material, accentuating contrasts, highlighting intonation or sound frequencies, tapping a rhythmic beat. In this way, the teacher stimulates the creation of new mental categories in the learners.
In turn, they will repeat the same statement, and will then go back and forth between their attempts and the models proposed by the teacher, until the latter deems the pronunciation of the sentence “acceptable”… or not, as not everything is always achieved on the first try. 
An example of phonetic coaching using the verbo-tonal method
