Add voice messages to slack. Append this code at the end of the file /Applications/slack.app/Contents/Resources/app.asar.unpacked/index.js (this works on windows too)

add voice messages to slack - index.js

// node electron
const { remote } = require('electron')
const currentWindow = remote.getCurrentWindow()

// document.addEventListener('mousedown', function() {
//   remote.BrowserWindow.getAllWindows().forEach(function(win) {
//     win.webContents.openDevTools({mode: 'detach'});
//   });

//   remote.getCurrentWindow().getBrowserView().webContents.openDevTools({mode: 'detach'});
// }, true)

function inject() {
  remote.getCurrentWindow().getBrowserView().webContents.executeJavaScript("$('.msg_input_wrapper').length == 0", false, (result) => {
    if (result) {
        setTimeout(inject, 200);
        return;
    }
    remote.getCurrentWindow().getBrowserView().webContents.executeJavaScript(script);
  });
}

inject();

var script = `
$(function() {
    function uploadFile(file) {
        var fakeDropEvent = new DragEvent('drop');
        Object.defineProperty(fakeDropEvent, 'dataTransfer', {
            value: { files: [file], getData: () => {} }
        });
        Object.defineProperty(fakeDropEvent, 'shiftKey', {
            value: true
        });

        // temporary disable thread file feature
        var tmp = TS.boot_data.feature_file_threads;
        TS.boot_data.feature_file_threads = false;
    
        document.body.dispatchEvent(fakeDropEvent);
        setTimeout(() => {
          TS.boot_data.feature_file_threads = tmp
        });
    }

    let recording = false;
    let mediaRecorder;

    function recordAudio(e) {
      e.preventDefault();
      if (recording) {
        recording = false;
        $('#recordAudio').css('color', '');
        mediaRecorder.stop();
        return;
      }
      recording = true;
      $('#recordAudio').css('color', 'red');
      
      navigator.mediaDevices.getUserMedia({ audio: true }).then(stream => {
        //const options = { mimeType: 'audio/wav' };
        const options = {};
        mediaRecorder = new MediaRecorder(stream, options);
        mediaRecorder.start();

        const audioChunks = [];

        mediaRecorder.addEventListener("dataavailable", event => {
          console.log('data');
          audioChunks.push(event.data);
        });

        //setTimeout(() => {
        //    mediaRecorder.stop();
        //    console.log('stop')
        //}, 3000);

        mediaRecorder.addEventListener("stop", event => {
          var file = new File(audioChunks, "audio.wav", {'type' : 'audio/wav; codecs=opus', lastModified: Date.now()})
          uploadFile(file);
        });
      });
    }

    function inject() {
        if ($('.msg_input_wrapper').length == 0) {
            setTimeout(inject, 200);
            return;
        }
        $('.msg_input_wrapper').append(\`<button id="recordAudio" type="button" class="btn_unstyle msg_mentions_button" aria-label="Record Audio" tabindex="3" style="
            right: 68px;
            width: 30px
        ">
            <i class="ts_icon ts_icon_microphone"></i>
        </button>
        \`);

        setTimeout(() => {
          $('#recordAudio').off('click');
          $('#recordAudio').click(recordAudio);
        }, 500);
    }

    inject();
});
`;

Click on the microphone and start recording, click again to stop and send the message.