There seems to be a bit of confusion creeping in here!
The audio output of the Airport Express is in the mV range - nowhere near enough power to drive loudspeakers! That's why, using an audio lead similar to the one advised by Intell, it needs to be connected to an amplifier, (to amplify the voltage to a useable level).
The audio lead consists of a 3.5mm Minijack (a plug with three "rings" - for the right channel, left channel and earth connections), which is plugged into the AE. These are connected to two separate leads (one for each channel) terminated by RCA phono plus which are then plugged into the amplifier.
BTW Apple also sell these leads, even though, if you are handy with a soldering iron, they are dead easy to make up for next to nothing.
Hope this helps.