How to override PulseAudio ALSA files?

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Hey, yes, I do have a workaround for that:

Doing pulseaudio.overrideAttrs and overriding buildCommand to not do any building, but instead copying the already-built files from the original derivation with cp.

Here is a concrete example which shows this technique on pulseaudio (it should also work for other packages) and also handles split-outputs (-out, -dev and so on):

let
  # Trying to have pulseaudio forbid Chromium to adjust volume gain,
  # but so far none of these have been effective.
  # See https://askubuntu.com/questions/689209/how-to-disable-microphone-volume-auto-adjustment-in-cisco-webex
  puseaudio_with_mic_boost_disabled =
    let
      constantVolume = 40;
    in
    pkgs.pulseaudio.overrideAttrs (old: {
      # name = "pulseaudio-patched";
      name = "${old.name}-patched-without-mic-boost";
      # Instead of overriding some post-build action, which would require a
      # pulseaudio rebuild, we override the entire `buildCommand` to produce
      # its outputs by copying the original package's files (much faster).
      buildCommand = ''
        set -euo pipefail

        ${# Copy original files, for each split-output (`out`, `dev` etc.).
          lib.concatStringsSep "\n"
            (map
              (outputName:
                ''
                  echo "Copying output ${outputName}"
                  set -x
                  cp -a ${pkgs.pulseaudio.${outputName}} ''$${outputName}
                  set +x
                ''
              )
              old.outputs
            )
        }

        # Replace:
        #     [Element Capture]
        #     switch = mute
        #     volume = merge
        # by:
        #     [Element Capture]
        #     switch = mute
        #     volume = ${toString constantVolume}
        # The target file `analog-input-internal-mic.conf` should be determined
        # by the output of `pacmd list-sources` for the relevant microphone `index:`,
        # which for my microphone is:
        #     active port: <analog-input-internal-mic>
        set -x
        INFILE=$out/share/pulseaudio/alsa-mixer/paths/analog-input-internal-mic.conf
        cat $INFILE \
          | ${pkgs.python3}/bin/python -c 'import re,sys; print(re.sub(r"\[Element Capture\]\nswitch = mute\nvolume = merge", "[Element Capture]\nswitch = mute\nvolume = ${toString constantVolume}", sys.stdin.read()))' \
          > tmp.conf
        # Ensure file changed (something was replaced)
        ! cmp tmp.conf $INFILE
        chmod +w $out/share/pulseaudio/alsa-mixer/paths/analog-input-internal-mic.conf
        cp tmp.conf $INFILE
        set +x
      '';
    });

And then I can use it for my system using:

  hardware.pulseaudio.package = puseaudio_with_mic_boost_disabled;

(Note that the concrete thing that I’m trying to do here, forbidding Chromium WebRTC to adjust my volume, still doesn’t seem to be working, but it does override PulseAudio ALSA file contents successfully.)

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