I have read thru the posts on sneezing and would like to add some things to consider:
With no nasal discharge or bubbly eyes but sneezing think when have you wormed your chickens
lungworms, gapeworms can cause sneezing as the worm attaches to the trachea and the chicken is trying to clear them
in a severe case the chicken will cough up bloody mucus and occasionally will contain a flat red worm , I have seen them up to
2 1/2 inches long. I use ivermectin pour on for cattle, 5mg per ml use 6 mls per gallon of water and leave as only water source for 24 hrs. repeat in 7 days no longer than 10. I have used febendazole but you have to shake it several times a day and treat for 3 days straight. ml is the same as cc so you will need a syringe to draw it up to put in the water.
Once they have had a bad upper respiratory infection or lungworm infestation the bird may be a chronic sneezer as damage was done to the respiratory tract.
Often upper respiratory infections are viral, antibiotics will do nothing for these
if you have a bird that is not eating or drinking much because they feel bad mix gerber baby food, I use turkey in gravy, with vits, warm water and dip the birds beak, even if it is just the bottom beak in the mix and after they taste it they will usually suck it down. In fact I have never had one not to. I feed them several times a day until the eat on their own. Remember also that a chicken loves wet bread. good way to get fluids down them also.
One of the most dangerous viruses is LT laryngotracheitis which there is a vaccine for, one drop in the nose once a year and a bottle that will treat a huge flock is @ $25.00
Hope this is helpful,