To send live audio to a connected host, move the mouse into its open connection window. When you do so, the mouse cursor changes into a button showing a telephone receiver. When you press and hold the left mouse button, the cursor changes to an ear and the legend "Transmitting" appears in the connection window, indicating you're now sending live audio from your audio input port to that host.
If you double click the left mouse button, live audio output is sent continuously until you next single click in the connection window. You can thus double click in multiple connection windows transmit simultaneously to a number of hosts (assuming your network is fast enough to handle the additional traffic). If your sound hardware is full-duplex, this lets you make "conference calls" where several people make connections to one another and take turns speaking. If your network supports it, multicasting may provide a more efficient way to conduct conference calls than directly transmitting to multiple hosts.
You can also use the space bar to toggle transmission of live audio; if you don't have a mouse, use Ctrl+Tab to activate the desired connection window, then press the space bar to begin transmitting. When you're done, a second press of the space bar ends the transmission.
After you've gotten familiar with sending in the normal "push to talk" mode, you may want to try out voice activated mode, which switches between transmit and receive automatically based on the sound level from your microphone.