unlock = function(e) {
        // Create a pool of unlocked HTML5 Audio objects that can
        // be used for playing sounds without user interaction. HTML5
        // Audio objects must be individually unlocked, as opposed
        // to the WebAudio API which only needs a single activation.
        // This must occur before WebAudio setup or the source.onended
        // event will not fire.
        while (self._html5AudioPool.length < self.html5PoolSize) {
          try {
            var audioNode = new Audio();

            // Mark this Audio object as unlocked to ensure it can get returned
            // to the unlocked pool when released.
            audioNode._unlocked = true;

            // Add the audio node to the pool.
            self._releaseHtml5Audio(audioNode);
          } catch (e) {
            self.noAudio = true;
            break;
          }
        }

        // Loop through any assigned audio nodes and unlock them.
        for (var i=0; i<self._howls.length; i++) {
          if (!self._howls[i]._webAudio) {
            // Get all of the sounds in this Howl group.
            var ids = self._howls[i]._getSoundIds();

            // Loop through all sounds and unlock the audio nodes.
            for (var j=0; j<ids.length; j++) {
              var sound = self._howls[i]._soundById(ids[j]);

              if (sound && sound._node && !sound._node._unlocked) {
                sound._node._unlocked = true;
                sound._node.load();
              }
            }
          }
        }

        // Fix Android can not play in suspend state.
        self._autoResume();

        // Create an empty buffer.
        var source = self.ctx.createBufferSource();
        source.buffer = self._scratchBuffer;
        source.connect(self.ctx.destination);

        // Play the empty buffer.
        if (typeof source.start === 'undefined') {
          source.noteOn(0);
        } else {
          source.start(0);
        }

        // Calling resume() on a stack initiated by user gesture is what actually unlocks the audio on Android Chrome >= 55.
        if (typeof self.ctx.resume === 'function') {
          self.ctx.resume();
        }

        // Setup a timeout to check that we are unlocked on the next event loop.
        source.onended = function() {
          source.disconnect(0);

          // Update the unlocked state and prevent this check from happening again.
          self._audioUnlocked = true;

          // Remove the touch start listener.
          document.removeEventListener('touchstart', unlock, true);
          document.removeEventListener('touchend', unlock, true);
          document.removeEventListener('click', unlock, true);
          document.removeEventListener('keydown', unlock, true);

          // Let all sounds know that audio has been unlocked.
          for (var i=0; i<self._howls.length; i++) {
            self._howls[i]._emit('unlock');
          }
        };
      }
Some browsers/devices will only allow audio to be played after a user interaction.
Attempt to automatically unlock audio on the first user interaction.
Concept from: http://paulbakaus.com/tutorials/html5/web-audio-on-ios/
@return {Howler}