What's the fastest way to organize audios one after another in a composition?

Here's a helpful suggestion that sounds like it will take more time and be more work, but the opposite is true. You will save a lot of time in the long run.

After Effects is a visual effect, compositing, and motion graphics app specifically designed and well suited to creating shots that you cannot make in an NLE like Premiere Pro. It is a lousy video editing app but very capable and well suited to creating shots. When you understand the tool and start using it to create shots instead of movies, you will save a lot of time.

A compilation of songs will probably be several minutes long. Unless you add a single music visualizer that bounces to the beat, there will be many shots in the final movie. Do your editing in Premiere Pro, and use AE to create the shots you can not create in Premiere Pro (or your favorite NLE).

I edit movies all the time that have long sections of motion graphics or visual effects, and most of my comps are under 7 seconds because most shots in a film I edit are less than several seconds long. The only time I ever have more than one shot in a composition is when I have to transition between shots that I cannot do in my NLE. Many times, the comp is only as long as the transition. I have created comps that are only a few frames long then added the rendered effects shot to the timeline in the NLE. 

If you adopt this kind of workflow, you will spend less time fiddling, a lot less time rendering, making changes in a few minutes instead of a few hours or even days, and being more productive. You will have more assets (files) to manage, but that does not take much time.

Pretty much what Kyle said: Since AE is terrible at mixing audio, such a job would be best done in AU or PR and then importing a pre-mixed solid file with the point here being that likely you'l get scratches and drops in your audio if you do this in AE on such a scale due to how the audio will be sampled in realtion to the timeline rather than the native info in the files.

Mylenium

Jose's suggestion is definitely the best way to arrange these audio tracks within After Effects, but it's worth saying ... there are several other apps that would be MUCH better for dealing with audio! If you're doing any kind of mixing, volume adjustment, etc., you're going to very quickly realize that AE's audio capabilities are extremely basic. 

Depending on what you need to do, Premiere or Audition would likely be much better environments for this project. (You can even copy/paste all your tracks from AE to Premiere, so you don't have to start fresh!)