There are many differestn schools of thought when it comes to subjects such as this. Without having been there speculation is almost useless. Some say that before the fall there were many other dimensions that humans had available to them including the ability to communicate with animals!
The safest bet you can take is to go with the information provided. The text says that the serpent "said". Therefore I know he communicated to her the wrods that followed. I personally believe he spoke but again I'll have to run that by someone who was actually there when I get to heaven!