American English listening difficulty

Where can I find online discussion of American English speech patterns that create problems for foreign listeners. I am currently working with a Greek visitor to the US who is having trouble with understanding conversational English. He speaks reasonably well, but has a tough time when listening to others speak. I am not referring to phrasal verbs or idioms, but to the colloquial style of actual speech. He has stated that Americans often drop their voices at the end of a sentence. Is this common? Why do they do it? When they do this, he often does not hear/understand what is said. Are there other difficulties people have with listening to Americans speak? I am attempting to compile a listing of such problems listeners experience as well as help my friend.