a couple of possibilities come to mind, both of which are not offensive…
yes, the obvious explanation which most people jump to first is that these people are presumptuous and have egos equating them with the priest…but more likely it’s not as sinister…
a) they are recent converts from denominations wherein there is more active participation and they just haven’t got the message that our service is not like that. I picture them sitting at the computer complaining to their friends how quiet the rest of their new catholic congregation is, what an insult it is that more people do not throw themselves into the service
b) they are so caught up in the service they are not aware they are speaking laudibly. I know there are times I’m following the service so closely I find myself saying the priest’s part but it’s not loud enough for others to hear, though my kids will shoot me a look and then giggle at me.
Anyway, perhaps considering one of these other options may help you not get as irritated as quickly as before. Though, of course, after closer observance you may find these people really are presumptuous, and then you’d be driven to pray for them rather than get angry about it.
Good luck with that. It’s difficult, I know, but do you really think God takes offense when parishioners join in prayer with his priests? Somehow I think not, especially when you consider we are all joining the angels at saints with the Holy Holy Holy…who knows if they’re remaining silent at the appropriate moments??