Convert your audio to wav or aiff. AE, and PPro and FCP for that matter, don't play very well with highly compressed audio formats like MP3.
I did try WAV file but nothing shows up. I mean nothing, like there is not even a drop down menu. It's blank gray.
I tried with After Effects CS3 on an old computer with mp3 file and there was a drop down menu and was able to select it.
I'm not sure if this is an audio problem. What could cause a drop down menu not to show up at all?
You haven't said what version of After Effects or what operating system.
That is important information.
After Effects CS4
Mac OSX 10.5.8
it works fine on
After Effects CS3
Mac OSX 10.4.11