As some of the comments hint, the link is broken (de|en should be de|de); it translates from German to English. (The beatboxing "hack" only works with German-to-German translation.)
Ok so that's weird. I was just showing this to my sister earlier today and noticed that there was no Listen, just Beatbox.
Naturally I was delighted that they recognised the meme ... but, talk about a race condition. I thought this was nothing new and here were are, a 48 minute old HN submission whereas I was showing my sister probably 50 minutes ago.
Slightly related, the first hit on google for "google beatbox" is translate.google.com
Probably because the Google Crawler registered all "google beatbox" links leading to that page.
People have used this feature before to "fake" results, although this time it is clearly legit. It's a shame Google doesn't register hash (#) locations.
The thing that really surprised me is that at first someone made a beatbox with MISUSING Google Translate! but now google instead of (for example) banning that misusing, has created a tool to help you enjoy Google Translate in any way you like! even if it's not it's original purpose!!
The easter egg is a simple 'beat box' icon below the original text much like the 'listen' icon under the translated text.
Edit: somebody in the thread uploaded a screenshot, thx: http://i.imgur.com/4jd5y.png