To avoid mixing down to mono or stereo, choose Edit > Preferences, choose “Import / Export” on the left, click the radio button “Use custom mix…” then click OK.
There is of course MP3 “Surround Sound” ( BBC NEWS | Technology | MP3 surround sound system debuts and MPEG Surround - Fraunhofer IIS ) which is still 2-channel but encodes spatial information for the extra channels that can be interpreted by MP3 Surround players ( Downloading Fraunhofer MP3 Surround 2.0 ).
If you want MP3 “Surround Sound” and are able to play it, then after changing the Audacity Preferences as above, export as “WAV (Microsoft) signed 16 bit PCM”. Then download the “Demo Software mp3 Surround Encoder” for Windows from http://www.iis.fraunhofer.de/en/bf/amm/download/software/download/download.html . The encoder is easy to use - just drop the exported WAV file onto the encoder. Note that “evaluation” encoder does not support more than 6-channel WAV files.
You can also use the command-line version of the evaluation encoder in Audacity itself. When you export, choose “(external program”) then click “Options…”. There is some help with exporting at the command-line in the Manual .
In your command you have to include options for the input bit depth (-res which must be 16-bit or 24-bit, so change the Audacity Quality Preferences if needed), channels (-c), sample rate (-sr which is the same as the Audacity project rate) and the output bit depth (-br). So the command will look something like this:
"C:Program Files (x86)mp3senc_win64_fcrv15_20080530mp3sEncoder.exe" -if - -br 192000 -c 6 -res 16 -sr 44100 -of "%f"
Gale