I answer to myself,

The trick or workaround is to use timer.performWithDelay

This works if you are using sounds with the same lenght.

Here is the code I'm going to use in our second Corona SDK Tutorial at www.macsoluciones.com (a kind of "relaxing sounds" app for the iPhone ;-) )

--- code ---
local ui = require("ui")

display.setStatusBar(display.HiddenStatusBar)
local dst = display.newImage("fondo.png")
local src = display.newImage("fondob.png")

local reproduce = function (event)

media.playSound(event.id)

end

local transicion = function()

transition.dissolve(src,dst,2000,1000)

src, dst = dst, src

end

local acampada = ui.newButton {

default = "buttonRed.png",
over = "buttonRedOver.png",
onEvent = reproduce,
text = "Acampada",
id = "acampada.mp3",
emboss = true
}

local bosque = ui.newButton {

default = "buttonRed.png",
over = "buttonRedOver.png",
onEvent = reproduce,
text = "Bosque",
id = "bosque.mp3",
emboss = true
}

local cascada = ui.newButton {

default = "buttonRed.png",
over = "buttonRedOver.png",
onEvent = reproduce,
text = "Cascada",
id = "cascada.mp3",
emboss = true
}

local grillos = ui.newButton {

default = "buttonRed.png",
over = "buttonRedOver.png",
onEvent = reproduce,
text = "Grillos",
id = "grillos.mp3",
emboss = true
}

local lluvia = ui.newButton {

default = "buttonRed.png",
over = "buttonRedOver.png",
onEvent = reproduce,
text = "Lluvia",
id = "lluvia.mp3",
emboss = true
}

acampada.x = 160
acampada.y = 80

bosque.x = 160
bosque.y = 160

cascada.x = 160
cascada.y = 240

grillos.x = 160
grillos.y = 320

lluvia.x = 160
lluvia.y = 400

timer.performWithDelay( 7000, reproduce,0 )
timer.performWithDelay( 20000, transicion,0 )

--- end of code ---