Yes I should have mentioned this - if you tried AAC multi-channel export using “M4A (AAC) Files (FFmpeg)” or “Custom FFmpeg Export”, it will fail because the default libvo-aacenc AAC encoder which the “recommended” FFmpeg uses only supports stereo.
And the standard MP3 specification only supports stereo.
WAV (very large) or AC3 export will give you a multi-channel file.
Alternatively, you can use the FFmpeg native AAC encoder instead which supports multi-channel export if you don’t mind exporting using “(external program)” and typing out a command: Problem exporting to AAC… Help! - #12 by Gale_Andrews
Gale