I’m cheating a big here as I’ve copied this from something I wrote on my blog but I agree wholeheartedly, signing does not delay speech as long as speech is always used in conjunction with the sign. Here is what I wrote on my blog which you can check out at earlylearningdownunder.blogspot.com
We made the decision to teach BC sign language before he was born. As an audiologist, it is my job to support the auditory/verbal mode of communication through devices such as hearing aids and cochlear implants, but in the case of hearing babies, I do believe that it is a fantastic complement to their auditory/verbal communication.
Research has shown that hearing babies of deaf parents started to communicate well before the hearing babies of hearing parents as they were learning both speech and sign language. Children have the ability and the intent to communicate long before they can appropriately verbalise their wants and needs in a way that the parents can understand. Therefore the introduction of some basic sign language can bridge the gap between wanting to communicate and being able to vocalise.
Hope that helps!