Hi Willa, I can't believe I didn't spot this post earlier.  I haven't ever seen this question discussed.  The similarities between those two creation accounts had never occurred to me before, but any similarity is probably not just a coincidence.  Not that either author copied the other or that together they invented the idea of a "sung" creation.  But they were both influenced by many of the same sources (like the Norse and other western European myths which they loved, and in which the power of the sung word is a frequent theme).  And at meetings of the Inklings this is exactly the sort of idea they would have talked about - from theological, literary, and mythological angles all at once.  So I would say that it was a result both of their friendship, their shared influences, and their like-mindedness on so many things.  But neither of them would likely have said that he got the idea of a sung creation solely from the other.
 
The power of the sung word appears elsewhere in Tolkien and Lewis's writing as well.  I can think of Tom Bombadil in LOTR, for example.  Pretty sure there are other instances in the Narnia books and maybe also in the space trilogy.  
 
The creation account in Ainulindale always moves me - particularly the part where Eru Iluvatar (God most high) responds to the rebellion of Melkor.  That is such an awesome word picture on the question of God's sovereignty and the existence of evil.
 
I love the Silmarillion - maybe even more than LOTR, although it is kind of an acquired taste.  Very different from LOTR, written in high epic style and much less like a modern novel.  If you read a good translation of Beowulf after reading Silmarillion, you will be struck by how similar in style they are - at least I thought so.  Let me know if you were able to finish it, and what you thought.