Get some Fun Tak (blue stuff) from an art supply store. Take a small wad of it the size of a piece of bubble gum, and press it into the speaker holes then slowly peel it out. Do it a few times and voilà, no more dust. You could also probably use silly putty.
Air can is too strong. It might damage your tiny speaker and mic. I'm using a static dust cleaner (the one use for DSLR camera) and rocket blower. It works really good for cleaning speaker, mic, dock port, headphone hole, etc.