K.B.
My son did this as an infant, and still does, and he's 5 yrs old. They're just exploring their vocals and using their imaginations. I left my sons mobile on his bed til he was about 3 yrs old. He would wake up and hit the button to listen to Mozart, Beethoven, etc... Then he would lay back down n put himself back to sleep. He would sleep until 10 am sometimes. But we used to laugh just listening to him on the baby monitor babbling away as the music played. I like to think he was singing himself back to sleep. :0)
And I bet he would still play that mobile if I left attached to his big boy bed.
Don't worry, she'll be fine. You'll find her saying more n more words that you didn't think she knew. I bet she's practicing saying them as she lays there. ;0)