I know that there are some spoken spells in Fantastic Beasts, but they are very limited. For example, Percival Graves seems to speak no spells at all, using his hands very often. Is there are specific reason for this?
In Fantastic Beasts, we only see adult wizards, the majority of whom are MACUSA employees, including aurors.
What we are seeing is non-verbal magic, which is taught to Hogwarts students in year six. Presumably, it's also taught at Ilvermorny or other American schools of witchcraft and wizardry.
‘... you are, I believe, complete novices in the use of non- verbal spells. What is the advantage of a non-verbal spell?’
‘Your adversary has no warning about what kind of magic you’re about to perform,’ said Hermione, ‘which gives you a split-second advantage.’
— Harry Potter and the Half Blood Prince
Percival Graves, because
he is Grindelwald in disguise, is one of the most powerful wizards of his time. His knowledge of magic goes beyond that of normal wizards or witches, which can explain away his use of hands in his magic as well as his ability to fight ten or more MACUSA wizards at once (while appearing to be winning on top of it).
Aside from Graves, Newt Scamander, the rest of the wizards we see use magic in the film are members of the MACUSA, specifically in law enforcement positions, so they should be quite skilled in the use of non-verbal spells.
The teaching of nonverbal spells was never covered in the Harry Potter films. But after the fifth film, it almost becomes universal among the older students and adult wizards. However, more powerful spells tend to need the assistance of the spoken word. e.g. the protection spell used by the teachers to defend against Voldemort.
Since the cast of Fantastic Beasts are all beyond school age there is no need for them to use verbal spells unless it is key to the plot. e.g. Queenie using Alohamora (Just so you know what spell she did!) I think there is also an assumption that the audience is now used to the effects of certain spells, so when you see the effect you know the spell that goes along with it.
Skill in nonverbal magic might be more important in American wizarding society.
I find it difficult to believe that the difference is because they are trained adults as opposed to students. While Harry and his friends are all students, they're certainly not the only ones who cast spoken instead of nonverbal spells. We see many competent, and even great, adult wizards over the course of Harry's adventures.
Most of the adult British wizards we know speak their spells, including some powerful ones in both the Aurors and the Death Eaters. I also don't think that American wizards are just objectively "more magical" those in Britain. Even if the wizarding schools in America are better, I highly doubt that every Auror in MACUSA is as powerful as Dumbledore.
However, American wizarding society is clearly more secretive than their British counterparts. They require all wizards to have a permit in order to be allowed to carry a wand, they forbid wizards having relationships with non-wizards, and the MACUSA has a map tracking every single spell cast in America.
A result of having to work harder to hide all aspects of wizardry might be that there is a greater focus placed on nonverbal spells in the American wizarding schools. Eliminating the need to say spells out loud would enable American wizards to cast them more subtly and attract less attention to themselves. In Britain, learning to cast nonverbal spells might only be considered useful to wizards who are likely to duel, and could use the advantage to catch the opponent off guard.
Newt isn't American, but might have to be quiet so he doesn't scare his creatures.
So maybe American wizards. Newt isn't American, so why wouldn't he speak his spells like other British wizards do? He's a magizoologist with an entire suitcase full of magical creatures. Some of these creatures can be very dangerous or even deadly if they get scared and think they need to attack. Newt could have learned how to do nonverbal spells so he could do magic around his creatures without having to shout spells and risk startling them.
Out-of-universe, it's probably a stylistic choice, or the creators not wanting to invent new spells.
What I've come up with above is the best in-universe explanation I could think of, but it still doesn't explain everything. If the reason that British wizards weren't as good as American wizards at using nonverbal spells, I'd think more of the people who knew they would likely be dueling a lot would have decided to learn it.
In the seven books, only the most talented duelists even have the ability to cast nonverbal spells. Even so, the ones who can don't do it all the time, which seems to suggest that it's easier to cast spells if you say the words out loud. Otherwise, why would anyone who could cast nonverbal spells ever say their spells out loud?
Out of universe answer :
In the first Harry Potter movie, spells were sometimes launched with the formula (wigardium leviosa) or sometimes not (Hagrid uses magic but does not pronounce any spell).
In the movies 2 and 3 we keep this strange case of the two cohabiting.
In and after the fifth movie , including Magic Beasts, all directed by David Yates, we see choreographed fights. With a LOT of spells being cast silently. Only major spells, or spells important for the spectator comprehension are spoken. Probably because it's more impressive.
So it might just be Yates going on with what he started, only major spells are spoken.