The spatializer effect automatically adopts a speaker channel format that matches the surround speaker mode of your currently selected platform. Presumably, the projects you worked on in the past had platforms set to use the “Surround 5.1” or “Surround 7.1” surround speaker modes, and your new project’s currently selected platform is set to use the “headphones” surround speaker mode.
To change the surround speaker mode of a platform, open the preferences dialog, select the “Build” tab, locate and select the platform you want to change in the “project platforms” list, and change the “surround speaker mode” property to something other than “headphones.”
