Examiner.com has a nice article about this:

There are a number of possibilities. For starters, we know that Bella (Kristen Stewart) is expected to experience hallucinations throughout the whole film. Resulting as a consequence of her thrill-seeking behavior, Bella is to resort to dangerous tasks in order to summon up an audio memory of her beloved Edward’s warning and protective voice. Perhaps, the director will use this opportunity to present these visions of Edward. Instead of a voice, they might present a vision of him. Or, maybe, alongside the voice, Bella could remember those piercing gold eyes or that crooked smile we all love.

Read the full article here.