To get audio-only you will need to rip the audio from the CD or from the mp4. I think if my memory serves me I have ripped an audio once by using VLC media player. See if your distro has installed it already.
I assume you are using Ubuntu (since you did not post what your distro is). In ubuntu you can install VLC using Synaptic ( click System -> Administration -> Synaptic Package Manager) then on the Search niche type 'vlc' click seach. If not found you may update your repositories % sudo apt-get update, then search again until you are able to install. You may also install the vlc mozilla plugin for your firefox if you like.
You can also extract audio from a video format using 'mplayer'. To shorten this post you may read the simple commands here
. You can tune the output to a finer bitrate if you like.
You can also use streamripper here
Hope this will do. Good luck.