Audio hardware on the Sun and Silicon Graphics machines which run Speak Freely for Unix is full-duplex but, unfortunately, many inexpensive sound boards installed in Windows machines are half-duplex, intended for "recording" and "playing" like a tape recorder instead of real-time conversation.
Speak Freely copes with half-duplex audio hardware in the following manner. When launched, it immediately attempts to open both audio input and output simultaneously, input first. If the output open fails, but then succeeds on a second try after input has been closed, the hardware is marked half-duplex. You can see whether Speak Freely detected your hardware to be half- or full-duplex by displaying the Help/About Speak Freely... or Help/Extended Status... dialogue boxes.
If a half-duplex card is installed, pressing the mouse button to send live audio immediately mutes any sound you're receiving and discards any that arrives while you're talking. When you release the mouse button, output of sound from other hosts resumes. You can send sound files to hosts even while they're transmitting to you but, of course, if their own hardware is half-duplex they won't hear it.
If your hardware is half-duplex, use the double click feature with great care. If you accidentally leave a connection in constant transmission, output will remain muted and nobody will be able to speak to you. If an input-output conflict has caused your audio hardware to be treated as half-duplex, an indication to that effect appears in the Help/About Speak Freely... dialogue box.
Since they all have full-duplex audio, some workstation users of Speak Freely for Unix transmit continuously. If you're talking to somebody who does this, consider asking them to indicate, perhaps by saying "Over", when they're done speaking and expect a response from you and, ideally, use the push-to-talk mode provided in Speak Freely for Unix when communicating with you.