In an open space environment, when your coworkers speak loudly it is useless to suffer silently, but none the less, do not flare up shouting “Can you shut-up?”.Bring up the issue tactfully, and try to ask nicely:
“Could you please lower your voice?”
“Please, make a little bit less noise. I know you probably don’t realize it but it is really disturbing”
“I have trouble focusing while you are talking. I’d really appreciate if you could talk a bit further away”
… Usually, people do not realize how loud they speak, so they will probably appreciate an amiable reminder. Do it with a smile and it is almost certain that no one would be offended. Be direct and tactful, but never attack personally.