If you upload a video to YouTube, then download it, then re-upload it, some changes in both the video and audio happens. This is a fact of life given that the MPEG encoding system is normally "lossey".
This is version 1
This is version 1000
Side note, only for web design people. Check the for the closing tag of the embed tags that YouTube give you when you do the "copy embed code" thing directly off of YouTube. It seems that the close embed tag is missing most of the time. If you get it off some one that has embedded it them self it seems to close. Odd, and a real pain on Blogger where it bitches at you if your tags aren't closed.