Hi sheri,
I would'nt be too worried about it at the moment as probably just a phase that your little girl is going through, and you say she has just started doing it.......if you're concerned about it and it continues for, say, another couple of weeks or so, I would call your GP or HV and discuss it with them for advice and how they can help.
Mrs Bear is in her 40's and she still mumbles away, on occasion, in her yank voice.....think she was dreaming about Desperate Housewives the other night....swift, discreet kick soon shut her up :0)
Best of luck, sherrard and may I say hope you have a very Happy Christmas to you and all the family :0) x