Clicking to select a box when another is already selected will link the boxes if they are not linked, or remove the link if they are linked.

Adjust the Output Channels slider as desired to change the number of output channels.

The output channels are shown in gray boxes.

The tracks in your project are shown in blue boxes.The tracks from your project shown on the left will be mixed down into the output channels shown on the right: Currently there is no support for exporting multi-channel files when using File > Export > Export Multiple.To include only specific tracks in the exported file, select those tracks then use File > Export > Export Selected Audio., or mute the tracks you do not want to export.AAC multi-channel export is subject to the FFmpeg encoder used, as described in our file formats comparison table.

and have chosen the name and format of your exported file, this dialog appears which presents you with the number of tracks you have in your project and allows you to map these as desired to each of up to 32 output channels ( WAV, AIFF or OGG) or 8 output channels ( FLAC). When you click File > Export > Export Audio.

Mixing in audacity