ffmpeg is a command line program for transcoding multimedia files. You could e.g. convert mp4 to mp3.
Clone this repo:
$ git clone --depth 1 git://github.com/Ph3nx/ffmpeg.git
Create new app and add some config vars:
$ heroku create --buildpack https://github.com/Ph3nx/heroku-null-buildpack.git
$ heroku config:set LD_LIBRARY_PATH=./lib
Test to convert a .mp4 video to .mp3:
$ ./ffmpeg -i video.mp4 -vn -c:a libmp3lame -ar 44100 -ac 2 -ab 192k audio.mp3
Use included shorthand program, converts file named name.mp4 in name.mp3:
$ ./ff name