If you try to upload to youtube an audio file you recorded using your smartphone it will not accept because it is not a video. Then the solution is “converting” this audio in video, you can do it using a still image file as backgroung.
I used this command to do that:
$ ffmpeg -loop 1 -shortest -y -i imade.jpg -i audio.3gp -acodec copy -vcodec mjpeg result_video.avi
You can find more information about similar commands here:
http://ffmpeg.org/trac/ffmpeg/wiki/Create%20a%20video%20slideshow%20from%20images
