http://en.wikipedia.org/wiki/The_Farewell_Sermon
Audience - The Muslim people (specifically those on pilgrimage in 632 AD)
WATCO - obeying Allah's commands received through Muhammad?
Goal - To convince the people to continue to obey and follow Allah's commands which they have received through Muhammad.
How and STAR
Ethos - Muhammad is the prophet of the Islamic faith. He automatically has authority given to him from Allah. However, Muhammad still quotes Allah, and His will to the Islamic people of that time. This creates a lot of appeal to the people, being able to know the will of their God. By doing this, Muhammad creates more trust and more credibility for his people.
S - These people where expecting something to come from Muhammad, and he gave them everything that he felt necessary to leave with them. He had left many manuscripts, which were then compiled into the Qur'an. For him to say something, it is enough that it is asked to his people, because he is the prophet of their god. So for his audience, Muhammad provided sufficient explanation.
T - Any practicing Muslim would believe and heed the words of the prophet Muhammad. There is no doubt that this is something a normal Muslim would believe. The fact that it is Muhammad speaking automatically means that it is the prophet's voice and it should be heard.
Effective - This was incredibly effective for the first few years after Muhammad's death, however it was soon forgotten, and the people began to fight amongst themselves again.
So for those there at the speech, yes it was very effective, for those not present at the speech, it was not so effective. Sometimes this happens to speeches, where when you hear it you are moved much more than when you read it. Voice can sometimes do something that the written word cannot convey.