Well, one possibility is that Naruto's fighting style might change. The Sound Four have their own unique jutsu and combat techniques. If Naruto joins them, he could learn some of those, like how to better use sound - based attacks perhaps. He might also become more stealthy as the Sound Four are often involved in sneaky operations.
Well, if Naruto joins the Sound Four in fanfiction, it could lead to a complete change in the story's dynamic. Naruto's positive and determined attitude might influence the Sound Four to question their loyalty to Orochimaru. Maybe they would start to see a different path, one that doesn't involve all the dark and evil deeds Orochimaru has planned.
One possibility for the antagonists could be a rogue group of ninjas who are after the power of the Sound Four. They might see the Sound Four as a means to gain more influence in the ninja world.
Well, it could be that Naruto has to face some powerful enemies while trying to rescue the Sound Four. Maybe they got caught by an even stronger group that wants to use their powers for evil. Naruto, being the hero he is, would use his signature jutsus like the Rasengan to fight off the bad guys and free the Sound Four.
In such a fanfic, Naruto might use his leadership skills to reform the Sound Four. He could lead them to do good deeds instead of their previous evil ways. For example, they could help protect the Hidden Leaf Village from other threats. Naruto's positive influence might make them see the value of friendship and loyalty to a cause greater than themselves.
His character might become more complex. He would be exposed to the darker side of the ninja world more directly. He could develop a more cynical view while still holding onto his core values.
His character could become more morally ambiguous. Naruto has always been a symbol of good in the Naruto universe. But with the Sound Four, who are associated with Orochimaru, a rather evil character, Naruto might start to make decisions that are not so clearly right or wrong.
The relationship would likely start with a lot of mistrust from the Sound Four. But as Naruto shows his determination and fairness, they would gradually respect him. For example, Naruto might share his dreams with them, which could inspire the Sound Four to follow him more wholeheartedly.
If Naruto joined Sound in a fanfic, it would completely change the Naruto universe. His relationships would be flipped. Instead of being with Konoha friends like Sasuke and Sakura, he'd be among Sound ninjas. His training might take a different turn too, perhaps learning more brutal and underhanded techniques. This could also lead to a different set of battles and power dynamics in the story.