The Focus on the Family Radio Theater adaptations is actually faithful to the books (probably one of the most faithful we’ll have). Sure, there were some tweaks and some things were added, but it is overall faithful.
As for additions, some of it didn’t make sense. For instance, in LWW, they’ve added where Edmund follows Lucy into the wardrobe during a game of hide and seek, he actually has a conversation with her before going in-
Edmund: Oh, Lucy, what are you doing in here?
Lucy: I'm looking for a place to hide.
Edmund: Going back into the old wardrobe, are you? Susan will never find you in that country of yours.
Lucy: Stop teasing me.
Susan: Ready or not, here I come.
Edmund: Shh. I think I hear Susan coming. Hurry. Get in.
Lucy: Don't be cruel.
Edmund: I'm not. Now go in. I'll follow. Hurry before she finds us.
Lucy: Well, all right. But don't close the door.
Edmund: Of course not. Besides, if I close the door, then won't it block out the light to your magical country? There's an idea. Let's close
the door and see if your country's thrown into darkness.
In the book, Lucy doesn’t even know that Edmund was following her. So an addition like that doesn’t make any sense.
Though, there were some that actually worked well. In LB, they actually added dialogue where Tirian and Jewel give themselves up to the Calormenes:
Narrator: They turned and walked back together, shedding bitter tears.
Tirian: Calormenes!
Rishda: It’s him! The murderer! Kill him!
Tirian: Wait, wait, I surrender.
Calormenes: (randomly) What? It’s a trick!
Tirian: It’s no trick! See how I hold out my sword with hilt towards you? I who was King of Narnia and am now a dishonoured knight give myself up to the justice of Aslan. Bring me before him.
Jewel: I also give myself up.
Rishda: If that is true, then bind them: his hands and a rope around the unicorn’s neck! And I will have your sword, king! Now, what is that upon your head?
Tirian: A golden circlet!
Rishda: I will have that as well! Now come with us!
Adding dialogue there actually really added to the story while staying faithful to the books. It made Tirian's surrender feel more immediate and tense—like you're right there in the moment with him. Then Jewel also giving himself up shows that loyalty and deep friendship between them even in despair. Then by having Rishda actually speak, in a way, it kind of adds to his character as greedy and power-hungry—taking Tirian's sword and circlet—which sets him up as a villain more clearly than in the book. I mean, what CS Lewis wrote in the book is great, but this adaptation adds a layer of immediacy and drama that really works for audio. It makes the scene feel more alive and less like narration.