I got it working !!
All I had to do is adjust the number of channel.
I've set FFMPEG to output mono audio and it worked like a charm. Here is my new FFMOEG command:
var ffmpeg = child_process.spawn("ffmpeg",[
"-re","-i",
"A.mp3",
"-ac","1","-f",
"f32le",
"pipe:1" // Output to STDOUT
]);