When you upload a video to YouTube, they don't store a perfect digital copy of your media file. The video is actually re-encoded into the MPEG-4 video format. This saves space on YouTube's servers, but also introduces some random degradation to the video (as a result of the lossy compression process).
So what happens if you then save that converted video, and upload it to YouTube again? The video (and audio) is degraded a little bit more. And what if you repeat that process 750 times? Here's the result: