Well when he barks and pees on your leg, he is saying OUT, and if he barks twice if it is a smart dog it might mean number two, and open the door quicker. If he brings his dish drops it on your foot and barks, he's hungry, etc.
What a great idea for a hub! I agree LRobbins that it's mostly in the eyes and the body language. My chihuahuas are vocally epressive as well. One has a special "I want to go outside" bark. It's a low "woof" that is funny to hear coming out of a tiny chihuahua's mouth. Another howls when I get off schedule. If it's time to make dinner and I'm not in the kitchen he will go to the kitchen and howl, and if it's time for bed and I'm not making any movement in that direction he will sit on the bed an howl. He's a little creature of habit. Now that I think about it they have lots of verbalizations that they have trained me to respond to. They are so cleaver!