I don't think AAC and MPEG4 were designed to be applied twice. It could be better if Youtube would ask for a given format and not re-encode both video and soun

I don't think AAC and MPEG4 were designed to be applied twice. It could be better if Youtube would ask for a given format and not re-encode both video and sound systematically. The situation for sound is particularly bad, and people deal with it by mastering "for Youtube" (ie: no basses).