Since you are seeing no other symptoms - just sneezing, check to make sure there is no mold/mildew and the they area(s) they are living in have adequate ventilation. Some respiratory irritations include mold, dusty feed and bedding, build up ammonia from feces due to poor ventilation, etc., so your problem could be environmental.
Without having other symptoms, it's hard to know what the cause is, you can try something like VetRx to see if there is any improvement. Antibiotics, imho, would be the last resort since they have no symptoms that would be associated with secondary infection.
With Fowl Pox, it's possible there could be a little sneezing if it's the wet form, but you would see lesions/scabs (somewhat oozing) on the face, combs, wattles and most likely plaques/canker inside the beak. Dry form of Fowl Pox, is generally not associated with sneezing, but you would also see scabs on the face, combs, wattles. Both forms of Fowl Pox are usually transmitted by mosquitoes and is quite common in spring, summer and fall. Wet Form may need medications, but Dry Form will usually resolve itself in a few weeks.
MG is possible, but it usually presents with more symptoms as well - thick mucous from the nostrils and watery/bubbly eye discharge, facial and eye swelling, coughing, gasping and gurgling while breathing are a few additional symptoms you may see.
