How a composition can automatically fit to an audio?

I think with aerender command you have the option of specifying a start and end frame of the comp for the actual render.  You would need to calculate how many seconds the audio file is and then translate that into how many frames the composition should be according to the audio length.  You would need to know the composition's frame rate to calculate that amount of frames.  Then use that frame count as the "end" of where aerender should stop rendering.

Another option is to use the developer friendly extension, Templater for After Effects (http://dataclay.com/templater), to accomplish this.  It is licensed software, but it has a set of patented  techniques known as "Time Sculpting" that can auto-adjust the composition duration based on a layer that the user specifies.  With the correct configuration, each time Templater changes the source of the audio layer, it will change the duration of its containing composition.

See this documentation on it: https://support.dataclay.com/templater/content/concepts/time/time_sculpting.htm

More specifically you would want to read about the "Time Cropping" technique:

https://support.dataclay.com/templater/content/concepts/time/time_cropping.htm

Hope this helps!

You should create a script to do that