Hiphop music is a really bad place to learn english. Most artists can barely speak english; or at least choose not to. You don't want to go around speaking ebonics. To answer your question, someone with diamonds in his mouth is referring to their grill. They have so much money they can make themselves look stupid by putting braces on their teeth decked out with gold, diamonds or other expensive stuff. Boys talkin down is probably other guys insulting him. To talk down to someone is to be condescending or insulting. So together, my interpretation is that he's contrasting: These guys are insulting me, but I've got all this money, and therefore I'm cool.

Thanks, Cory! I must admit that the words completely foxed me, but I don't listen much to rap or hip hop. Now I understand a little!