Default to AAC audio for MP4 files

[allejok96]

Tried to open a video on a Mac and it had no sound. Took me a while to figure out the problem, as it was a presentation on someone else's computer. I'm not the first one... #51, #820, #876

Instead of h264/mp3 the default should be h264/aac. I cannot provide any source, but this seems like the common practice. At least it's the ffmpeg default, and Quicktime requires it. I see no reason to deviate common practice in this case (well, perhaps Debian, but both aac and mp3 are non-free (still, I think?) and if a Debian user wants, he can choose some other format to encode to).