He actually can talk, If you read his picture book it clearly shows that Sandy is able to speak. (Granted it's a picture book and the majority of the lines are the narrator, but Sandy does have a line.)

I'm good with either options, I find the use of sand in communication creative and it's an advantage of being able to highlight the personality more. Sandy's body language and facial expressions reflect his character, something that might be difficult since dialogue is usually used for this purpose. So, yeah.

But I'm really curious on what his voice sounds like.

I LOVE that Sandy doesn't talk. I think it's about time we have a character like Sandy. I really, really hope Sandy doesn't have a voice and that they don't give him one if they make a sequel. I like to think the Sandman is mute. It would be wonderful to have a mute character in a movie, and it's very fun and creative to see him communicate with his dream sand symbols.

yeah. it would be nice if he talked. :)