There are actually several was that you fcould phrase this sentence.
You could say:
I saw Elena smile and whisper something to her baby, but I could not tell what she was saying.
I saw Elena smile and murmur something to her baby, but I could not tell what it was.
You could use most of the synonyms that go along with the word whisper, but be careful so that you're sentence still makes sense and in grammatically correct, dependent on where you place these verbs.
