Well the first thing to do is to run the audio set-up wizard and make sure Express Talk is using the right audio device. Assuming the problem still exists, the likely cause is a firewall between your computer and the internet that is blocking the audio.
In most cases this firewall is built-into the router or broadband modem you have. What you need to do to fix this is forward the following ports on your router to the computer running Express Talk:
UDP 5060 and UDP 8000-16000
Now run the network set-up wizard in Express Talk, there shouldn't be any errors or warnings. Make a test call and it should work. If not, there is most likely another firewall blocking the connection and this is probably software based (e.g. Norton Internet Security, Windows Firewall, Zonealarm, etc.). Make sure you configure this software to allow connections to/from Express Talk (if necessary specify the ports listed above).
If all goes well your audio problems will be fixed.
