I too have noticed that the Africans who live here do talk loudly when together. It may be that we notice it more because we don't understand what is being said - or it may be that they are used to being in large groups, and do not use the standard communication courtesies of Western society - waiting for someone to finish speaking before you begin - so the only way you ar heard is to talk louder than anyone else. This is in the nature of an educated guess, rather than fact, so I'd be interested in an actual reason, if there is one.