I don't think most people intend to be rude; they simply don't think, or don't realize, how their words or actions affect others.

For example, have you ever been at a meeting, a class, a sermon, or a show of some sort, and been aware of a couple of women sitting near you, or perhaps two men standing directly behind you, all carrying on their own conversation, oblivious to anyone and anything else?  

It can be very frustrating, and you can be sure that you are not the only person affected by this lack of consideration - it's most likely bothering others in the immediate vicinity, as well as the speaker. (Yes, we do notice these things and find them just as distracting as you do.)

If you are a people watcher, you might notice the first reaction is a quick glance around -- who is making the noise? The second look is usually accompanied by a frown, and the third, a scowl. Clearly, it has gone from an irritation to being an annoyance and even provoking anger.

So what do you do?          

Depending on the situation, you may be able to find assistance via an event host or coordinator; let that person take on the task of shushing the offenders.  

Should you choose to speak to the culprits yourself, keep in mind the expression "You catch more flies with honey than vinegar" and frame your request in such a way that you change the perspective of the other person - make it about you, not their behavior.  

For example, a simple "Excuse me, but I'm having trouble hearing" may be all that is necessary. Another option I recently heard (and rather like) is "I'm sorry, I'm having trouble concentrating on the speaker and your conversation at the same time." As this is on the somewhat snarky side of things, be sure to include a big smile with this one!

If it comes down to it, you can always change seats at the break, or intermission.