UWP の連続音声認識のサンプルコード ( https://blogs.msdn.microsoft.com/shintak/2016/07/22/constraintrecognition/ ) に音声入力がタイムアウトした場合の継続処理を追加。Package.appxmanifest の Capabilities で Microphone にチェックを入れるのを忘れずに。

MainPage.xaml.cs

    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class MainPage : Page
    {

        public MainPage()
        {
            this.InitializeComponent();
        }

        //連続音声認識のためのオブジェクト
        private SpeechRecognizer contSpeechRecognizer;
        private CoreDispatcher dispatcher;


        protected async override void OnNavigatedTo(NavigationEventArgs e)
        {
            //ハックグラウンドスレッドからUIスレッドを呼び出すためのDispatcher
            dispatcher = CoreWindow.GetForCurrentThread().Dispatcher;

            //初期化
            contSpeechRecognizer =
                new Windows.Media.SpeechRecognition.SpeechRecognizer();
            await contSpeechRecognizer.CompileConstraintsAsync();

            //認識中の処理定義
            contSpeechRecognizer.HypothesisGenerated +=
                ContSpeechRecognizer_HypothesisGenerated;
            contSpeechRecognizer.ContinuousRecognitionSession.ResultGenerated +=
                ContinuousRecognitionSession_ResultGenerated;


            // 音声入力ないままタイムアウト(20秒位)した場合の処理
            contSpeechRecognizer.ContinuousRecognitionSession.Completed += ContinuousRecognitionSession_Completed;

            //認識開始
            await contSpeechRecognizer.ContinuousRecognitionSession.StartAsync();
        }

        private async void ContinuousRecognitionSession_Completed(SpeechContinuousRecognitionSession sender, SpeechContinuousRecognitionCompletedEventArgs args)
        {
            await dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
            {
                textBlock1.Text = "Timeout.";
            });

            // 音声を再度待ち受け
            await contSpeechRecognizer.ContinuousRecognitionSession.StartAsync();
        }

        private async void ContSpeechRecognizer_HypothesisGenerated(
            SpeechRecognizer sender, SpeechRecognitionHypothesisGeneratedEventArgs args)
        {
            //認識途中に画面表示
            await dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
            {
                textBlock1.Text = args.Hypothesis.Text;
            });
        }

        private async void ContinuousRecognitionSession_ResultGenerated(
            SpeechContinuousRecognitionSession sender, SpeechContinuousRecognitionResultGeneratedEventArgs args)
        {
            //認識完了後に画面に表示
            await dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
            {
                textBlock1.Text = "Waiting ...";
                output.Text += args.Result.Text + "。\n";
            });
        }
    }