I don't remember my dreams very well in general, so I don't remember hearing sound in them either but I'm pretty sure I can hear sound. Somehow I'm able to talk with people in my dreams, after all. When it comes to smell, I have no idea, I don't remember smelling anything in my dreams at all.

I think hearing and smell might just be things we simply don't remember well from dreams. We get the vast majority of information from what we see, so that's the first thing we'd remember. The only times I remember feeling something in dreams is when I got stabbed with needles, shot and that one time I dreamed that I smoked when I was a kid (even though I've never done that before). The reason I remember those is just that they left an impression of some sort. With hearing or smell, I think it just doesn't leave an impression or is special in any way, so we simply don't remember it.