Video is the heaviest thing most of us ever attach: screen recordings, camera clips, whole talks. It was born in wherever your camera, screen recorder or download came from, and its real weak spot is nothing inherently; every container brings its own quirks.
AAC is MP3's better-trained successor: cleaner sound from the same number of bits. Advanced Audio Coding: the default soundtrack of YouTube, iTunes and most MP4 video. That makes it the right destination when what you need is streaming-grade sound at friendly file sizes.