Setting the Default Output Bus (Main Mix)

The Main Mix is the default output bus to which each new audio, group, or FX channel is automatically routed. If only one bus is available, this bus is automatically used as the default output bus.

1. In the Audio Connections dialog, right-click the output bus that you want to use as default output bus.
2. Select Set <bus name> as Main Mix.

Result

The selected bus is used as default bus. The Main Mix is indicated by a speaker icon next to its name.