You can create L-C-R with the following tool:
https://forum.audacityteam.org/t/karaoke-rotation-panning-more/30112/1
Load your stereo file and duplicate it.
Use “Remove center” on the first track, this gives the left and right channel.
Use “Isolate Center” on the second track, this gives 2 x C.
You can split to mono and delete one channel.
If you create additionally a pure bass track, you’ll have a 3.1 configuration.
I haven’t ready in mind, where the bass is taken from, this would need some more insight into the codec.
There’s sometimes a 90 degrees phase shift introduced, mainly to encode the rear channels.