I'm sorry your bird may be having some issues, but it's extremely difficult to tell you much from your description. Usually if a bird is coughing/sneezing/wheezing it is not an indication of mites, but rather of an Upper Respiratory Infection, usually bacterial, sometimes fungal, or both, or of Pneumonia in advanced stages. There are many different types of mites, but in-general they have nothing to do with breathing or lung issues, but are rather visibly seen around the beak and on the face, on their skin, or on the underside of their feathers. So it's not mites, unless you can see them or have another reason to think so.
At this point I would say that since he has not made the sound again, it only happened once, hopefully he's not actually sick, but rather was just coughing something up or out of his throat/nares, was adjusting his crop, etc., or just making a bird noise. What you need to do from this point on is watch for further signs of illness:
-if he continues to cough/wheeze/sneeze, or show any other signs of an Upper Respiratory Infection or other breathing issue
-fluffs-up his feathers and keeps them fluffed-up when it's not cold
-becomes very lethargic and/or starts sleeping more than normally
-stops eating/drinking
-vomits
-goes to the bottom of his cage
If he exhibits any of these signs/symptoms, specifically if he again coughs/wheezes/sneezes or you see any fluid coming from his nares/mouth/eyes, then he most likely has a URI and needs to see an Avian Vet for a culture and the proper prescription medications based on the results; if he exhibits any of the other sighs/symptoms then he isn't feeling well and something is wrong and again, needs to see an Avian Vet asap.
As of right now you just need to watch him very carefully, and I would also separate him immediately from the rest of your birds into his own cage in a different room completely, and this is for 2 reasons: Because he may be contagious, but also so you can measure how much food he is eating and how much water he is drinking to make sure it's normal.
Hopefully it's nothing at all and he's fine, which it most likely is. But keep a very close eye on him for these signs/symptoms, keep him warm and separate from the other birds, and if you notice any of these things happening then right to the Avian Vet. But i'm going to assume that this was just a freak-thing, a little birdy sound/movement and he's fine at this point.
you can also post a photo of him for us to see...
