How to speed up / slow down an audio stream with ffmpeg

Use the "atempo" filter:
ffmpeg -i input.mp4 -filter:a "atempo=0.5" -vn output.aac

The above example will extract the audio stream from the input video, slow it down by 2 and reencode it into an AAC audio file.
The parameter for atempo must be between 0.5 and 2.0. If you want to slow it down even more, you can use the atempo filter more than once.
Eg. to slow it down by 4:
ffmpeg -i input.mp4 -filter:a "atempo=0.5,atempo=0.5" -vn output.aac