From the WebUI of the Lifesize Endpoint Icon - 600, you can control the Audio Settings.
Logon to WebUI of Icon 600 from your system browser (https://ICON_IP_ADDRESS)
Username/ Password: support
Go to Preference tab > expand Audio > drop-down Video call Output
Choose Phone - to get audio output on Lifesize Phone device.
Choose HD Out - to get audio output on Primary display transmitting audio through HDMI cable