It raises or lowers the volume of the voice that is speaking. The default is set to the maximum volume. The next option is the speed of speech. This is useful if you are having difficulty understanding the text being spoken, or if you are in a hurry and want the speech to go quickly. This option is not dependent on a modem and so is available for all implementations.
Pitch is another modem dependent option.
The final option, Lexicon, controls the way the speech engine interprets the text that it reads. This engine automatically pronounces letters that it comes across that are in all capitals. This makes it convenient for say, stock ticker symbols or other acronyms that you may need. However, if you wanted to pronounce the company name in place of the stock ticker symbol, then the Lexicon button is where that is handled. When all the parameters have been decided upon, the voice can be tested if you have a sound card and speakers.