I don’t think this kids boasting is funny…at all, but I have had the hardest time finding this sound on the Internet. I think the main reason I’ve had a hard time finding it is because it has no name. I’ve tried cartoon sounds. I’ve tried wah wah waaaah. I finally came across a good discussion of this where it is referred to as a muted trumpet or a sad trombone/trumpet. And, they also refer to the Debbie Downer sketch on Saturday Night Live. As you can tell from the video, it is that sound made (usually in cartoons) after someone is let down because something did not turn out the way they expected. When you hear it, you’ll get it.