In a case like this, I would exercise common sense as opposed to pondering manners. I will need all the concentration I need when something is on the line, and would be too focused on the match to talk. But, I will exchange pleasantries before and after the match. That is the least you can do from a socially acceptable standpoint in tournies.
The way I go about it is it depends on the person and relationships I guess. Some people can be as talkative and social butterfly as heck so I find it right to talk back when it happens. Sometimes I may just end up talking but get no replies. If I see someone being super serious and put their headphones on then that's about the only time I don't talk cuz they obviously won't hear me lol