Install Sound Open Firmware

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I have a Lenovo C940 and with the 5.7 kernel I only needed to add

  boot.blacklistedKernelModules = [ "snd_hda_intel" "snd_soc_skl" ];

  # This can be removed when PulseAudio is at least version 14
  # https://wiki.archlinux.org/index.php/Lenovo_ThinkPad_X1_Carbon_(Gen_7)#Audio
  hardware.pulseaudio.extraConfig = ''
    load-module module-alsa-sink   device=hw:0,0 channels=4
    load-module module-alsa-source device=hw:0,6 channels=4
  '';

and I now have working microphone and 2 speakers (not the bass boosters though sadly)