I've tried breeding mud and air and some other stuff but it doesn't work. I need proof, by the way - only answer if you got one or have a friend that has one.
You can breed a Sandstorm Dragon by mixing Air and Earth. Since Air and Earth are opposites, you need to use at least one hybrid. So you could mix Air + Mud, Air + Moss, Earth + Willow, etc. Note that as with any hybrid dragon breeding, you should be aware of pairs which can produce unintended long incubate offspring. In this case as BenBrocka points out Earth + Sonic can be a very bad pairing as it can produce the 24 hour incubating Crystal.
It has a two hour incubation period, so if you see something different than that you did not get it, but since other dragons you could breed also have two hour incubation, you have to wait to see the egg to be sure. From the wiki, the egg looks like this: