public function unpause()
    {
        $deferred = new Deferred();

        if (! $this->speaking) {
            $deferred->reject(new \Exception('Audio must be playing to unpause it.'));

            return $deferred->promise();
        }

        $this->isPaused  = false;
        $this->timestamp = microtime(true) * 1000;
        $deferred->resolve();

        return $deferred->promise();
    }