I've had chickens that sneeze - always chicks, though. It's never really been a problem and has always gone away. How old are your chickens and are you seeing any other symptoms?
My entire flock was eaten by a raccoon last week so.... I just brought these gals home yesterday... They are about 5-6 months old. I did put in some new wood chip horse bedding today - could that be it?? Thing is, the coop is aerated very well and all the girls are out free ranging in my yard just sneezing away! I have one particular nosey one, Eunice, that keeps following me everywhere and evey few minutes I hear it!