To change your server codec follow these steps;
1.) Login to the web control-panel: http://control.branzone.com
2.) Browse to: Voice Servers > YOUR_SERVER > Voice Server Configuration
3.) Once in the configuration page select to edit the "ventrilo_srv.ini" file.
You will then see the option to change your codec at the bottom of the options page. Once you have saved the changes you will need to restart your server.