1. The cogency of an argument is neither augmented nor diminished by the passion of the arguer. Cogency and passion are logically independent. The same goes for the truth or falsity of an assertion. The raising of the voice cannot transform a false claim into a true one, nor make a true one truer.
2. What's more, any display of a passion such as anger is likely to be taken by the interlocutor as a sign that one's argument is nothing but an expression of passion and thus as no argument at all. He will think your aim is to impose your will on him rather than appeal to his intellect. The interlocutor will be wrong to dismiss your argument on this ground, but you have yourself to blame for losing your cool and failing to understand human nature. If your aim is to convince someone of something, then you must attend not only to your thesis and its rational support, but also to the limitations of human nature in general and the particular limitations of those you are addressing. 'Tailor your discourse to your audience' is a good maxim.
3. While bearing in mind points 1 and 2, you must also realize that a failure to show enthusiasm and commitment may also work against your project of convincing the other.
4. 'Rhetoric' is too often employed pejoratively. That is unfortunate. The art of persuasion is important but difficult to master. It is not enough to know whereof you speak; you must understand human nature if you will impart your truths to an audience.