Yes, even as a stream. The author mentions that a streaming conversion of mp4 to mp3 would be very difficult/impossible. I actually built that exact system for a simple youtube->mp3 downloader. FFMpeg can perform a streaming mp4->mp3 conversion. See here: https://github.com/matttt/youtubesampler/blob/master/main.js...

It's not impossible, but it would be way less efficient as it need complete re-encoding of the audio, as my system just re-format the container format.

ffmpeg can copy streams without transcoding. For example: ffmpeg -i input.m4a -c:a copy input.mp4

In this case both mp4 and m4a (which are the same thing) use AAC. So no transcoding, but mp3 is not. So in the case of mp4 > mp3 it need a complete re-encoding of the audio.