1 - Le matériel et les pilotes ALSA, FFADO, ...

# cat /etc/asound.conf
pcm.dshare {
   type dmix
   ipc_key 2048
   slave {
       pcm "hw:0"
       rate 44100
       period_time 0
       period_size 1024
       buffer_size 8192
       channels 8
   }
   bindings {
       0 0
       1 1
       2 2
       3 3
       4 4
       5 5
       6 6
       7 7
   }
}
pcm.out12 {
   type plug
   slave {
       pcm "dshare"
       channels 8
   }
   # ttable syntax (as I understand it) :
   #   ttable.INPUTCHANNEL.OUTPUTCHANNEL VOLUME
   #   so in the example below, I redirect the first channel of the input to the first channel of the output
   #   i do the same for the second channel and I mute the others
   ttable.0.0 1
   ttable.1.1 1
   ttable.0.2 0
   ttable.1.3 0
   ttable.0.4 0
   ttable.1.5 0
   ttable.0.6 0
   ttable.1.7 0
}
pcm.out34 {
   type plug
   slave {
       pcm "dshare"
       channels 8
   }
   ttable.0.0 0
   ttable.1.1 0
   ttable.0.2 1
   ttable.1.3 1
   ttable.0.4 0
   ttable.1.5 0
   ttable.0.6 0
   ttable.1.7 0
}
pcm.out56 {
   type plug
   slave {
       pcm "dshare"
       channels 8
   }
   ttable.0.0 0
   ttable.1.1 0
   ttable.0.2 0
   ttable.1.3 0
   ttable.0.4 1
   ttable.1.5 1
   ttable.0.6 0
   ttable.1.7 0
}
pcm.out78 {
   type plug
   slave {
       pcm "dshare"
       channels 8
   }
   ttable.0.0 0
   ttable.1.1 0
   ttable.0.2 0
   ttable.1.3 0
   ttable.0.4 0
   ttable.1.5 0
   ttable.0.6 1
   ttable.1.7 1
}