Assuming video.mp4 has an audio stream encoded with AAC, the following command will stream-copy that stream to audio.aac. The -c:a copy parameter will instruct ffmpeg to copy the audio stream instead of re-encoding it. The -vn parameter will ignore the video stream of all inputs.
https://www.ffmpegbyexample.com/examples/0484nafv/extract_an_aac_audio_track_from_video_file/
Assuming
video.mp4has an audio stream encoded with AAC, the following command will stream-copy that stream toaudio.aac. The-c:a copyparameter will instruct ffmpeg to copy the audio stream instead of re-encoding it. The-vnparameter will ignore the video stream of all inputs.