I have no qualms with people blowing their noise in public so long as their snot does not make an aquaintence with my skin. I do not find it rude, and much better than having snot running down one's face (I hated how this was 90% of the kids I knew as a kindergartener...I still have nightmares). However, in England, it is generally considered polite to hold one's cough or nose blowing until a quiet break, such as a lull during a play. When over there, you will hear a huge cacaphony of withheld coughs during any pause. XD I like someone's suggestion of sitting closer so you can hear better. If truly disruptive, you could always lightly and happily ask them something like, "ah, poor thing, sounds like you have a cold. But oh my gosh, you are a super powerful nose blower! I would kill to have lungs like that, haha. Do you mind waiting for pauses when our prof is speaking though? That'd help me out a ton because I record his lectures, and my darn recorder picks up the sniffles and not his voice." No need to suffer in silence or lecture about etiquette in such a situation. Play it cooool.