I'll start this by stating that I am partially deaf. The only way I can tell how loud I'm talking is by the feel of my voice. I can't hear how loud I am. With the advent of iPods and I guess really, starting with the Walkman, loud music stuffed directly into the ears has caused a great deal of hearing imparedness in our culture.
That being said, while I can tolerate people being generally loud, I still can't handle hearing people argue in public. Or even in private. It sets off anxiety attacks and a distinct feeling of "need to hide".
Utopia, if you think you can do it in a calm and reasonable tone of voice, you might try telling the person that they're being loud enough to disrupt others and would they mind talking a little more quietly. I've found polite requests to work well, and it's entirely possible that the person doesn't realize how loud they're speaking (my husband has to ask me to not speak so loudly anytime I get excited, for good or for bad, about something as I honestly have no idea how loudly I'm speaking).