You need to have a microphone attached to your computer to pick up your voice. The web phone program you are running digitizes your voice message and sends it to the person you're talking to. The other person's computer undigitizes your voice and plays it. The audio quality is not great but you can easily make out what the person is saying. There is a slight delay while the voice messages are digitized and sent back and forth.
Sound quality seems to be improving with each new program upgrade.
Although still in refinement stages, this method actually allows you to see the person you're talking to. Click on the CU-See Me link to find out more information and to also download a free full working demo version good for one month's use.