Hearing a story is different than reading a story. It takes us outside our internal narrator and allows us to experience the story from a different part of our brain. No wonder you noticed new passages.

Yet that is also the richness of Tolkien. I've noticed that as I mature (Ha!) I notice different aspects of the story when I read it again.

Reading The Silmarillion will change the way you read the Lord of the Rings. A couple of pieces of advice on reading (or listening to) the Silmarillion:

It is not a novel. Tolkien liked to say that the Lord of the Rings was an account of the time as documented in the Red Book, and later translated into English. (Write what you know.) View the Silmarillion in the same spirit. It is Bilbo's recanting of the tales of the First Age, told by some of those that were there. Therefore, it is very condensed.

I recommend starting with the final chapter. It recounts the events of the Third Age in a few dozen pages. As you are familiar with the events and the characters, reading this chapter first introduces the reader to Tolkien's more scholastic style, which can take getting used to. Recognize that all of the Silmarillion is a condensed collection of tales.

Do not try to grok it all at once. Keep a map handy, plus the appendix of names, and the family trees, whether you are reading or listening, they'll help. But be willing to just hear the stories and let them flow, like you were sitting 'round a campfire in the wild.