Audio problems usually mean there's some traffic being blocked by the firewall. Are there any other external extension connected o this 3CX PBX that is working? If all external extensions are not working, maybe the correct ports (by default UDP 9000 - 9015) were not opened on the firewall on the PBX side. If yours is the only external extension not working, you could maybe have a firewall in your home network or computer that is blocking this traffic (are you using a personal firewall?).