He sings

 In the end of volume 15, we can see a characher riding a motorcycle and singing a song. This song doesn't look like any song I know (not une french one)

" We Are all dead, lalala" (3 times)

 "We are all in Heaven, lalala"

At first sight it's nothing very important, but in it's original version, this is a very very well known song. It's a classic japanse song, recognizable only with few words. It's "Kaettekita Yopparai" / The drunk who came back

This song goes back to 1967, time to the Urasawa's youth , and time to our heros youth too

This unknow character is simply whistelling a song from his childhood

The english lyrics are below

(I'm searching for the japanese one)

So by reading it, it doens't seem funny but I swear it's really funny and amusing!!!

It can be listenned and seen here : http://www.legnog.com/music/iol2x/

I wish to thanks very cordially, Funji-San from the forum, who gave me all this informations and also the links thru the flash animation

in italic the refrain and in fat the words of God

I died, I died, I died, 

And I went up to Heaven  

Long Staircase, Long Staircase made of clouds    

I climbed up very weakly        

I continue climbing step bye step, very weakly               

I, finally, arrived at the gate of heaven  

Heaven is a good place

Wine is good an chicks are cute  

The reason why i got killed.

Druk driving

I died, I died, I died,   

And I went up to heaven 

"Hey you, heaven is not an easy place to be! 

Be more serious"

Heaven is a good place, come at least once

Wine is good an chicks are cute

I keep drinking wine every day

I forget the words of God

"Hey you, still indulging yourself, wasting your time 

In that case get out of here"

That is the reason I was kicked of

That is the reason that I came down

I fall on the steps 

I woke back up in the middle of a field

I came back to life