Timer.cs

private readonly Timer _timer = new Timer(300) { AutoReset = false, Enabled = false };

// ...

_timer.Elapsed += (s, a) =>
{
    Console.Beep(1500, 200);
    _timer.Enabled = false;
};

// ...

private void OnDirectoryChange(object s, FileSystemEventArgs a)
{
    if (_timer.Enabled) return;

    _timer.Enabled = true;
    _timer.Start();
}