MediaRecorder.onresume = function(event) { ... }
MediaRecorder.addEventListener('resume', function(event) { ... })

...

  pause.onclick = function() {
    if(MediaRecorder.state === "recording") {
      mediaRecorder.pause();
      // recording paused
    } else if(MediaRecorder.state === "paused") {
      mediaRecorder.resume();
      // resume recording
    }
  }

  mediaRecorder.onpause = function() {
    // do something in response to
    // recording being paused
  }

  mediaRecorder.onresume = function() {
    // do something in response to
    // recording being resumed
  }

...