Select all your layers, go to Animation>Keyframe Assistant>Sequence Layers, check overlap, and put in negative 20 seconds for the amount.
And now for the caveat: I wouldn't use AE to work with sound.
The Good Mr. Szalam is absolutely right: just about ANY application is better for audio work than AE.
Okay, I'll exclude Microsoft Office; it would be worse than AE.
Audio in AE is good for reference and timing purposes. I'll occasionally use it because I really like the way the Reverb effect works: it's faster to get the right sound than anything else I've used.
It didn't turned out what I had hoped for.
I tried your suggestion, but it end up looking like this:
20 sec -> layer 1 (=15 sec.)
55 (40+15) sec -> layer 2
"I wouldn't use AE to work with sound."
Which other program would you suggest?
How about an audio application, for starters? Soundbooth comes with Adobe's Production Premium apps.
Or how 'bout Audacity, which is a freebie? There are a couple-three free audio applications, no matter your OS.
Or you could even use a video editing application! I do it all the time: Final Cut Pro. It's TONS easier than trying to bludgeon your audio clips into shape in AE.
Does that get you started?
I have windows 7 64.
Audacity is something I have but I've no idea how to merge the files together the way I want it, as described in my first post.
Sorry, I was thinking your sound clips were much shorter and that you wanted 20 seconds between them, I wasn't paying attention. This should do it.
Put all your layers at the beginning of your composition.
Select all the layers move the current time indicator (CTI) to one second.
Press alt+] to trim all your layers to one second long.
Do the sequence layers thing but put -19 seconds instead of -20.
All of your layers should now be starting at 20, 40, etc.
Press End to move your CTI to the end of the comp (make sure your composition is long enough that none of your audio layers will be getting cut off)
And press alt+] to bring all your audio layers back out to their full length.
Perfect! Thank you so much! And everyone else too.