Handling discrete volume per channel in the LilypadMP3

Just bought the Lilypad mp3 and assumed that, when using the SFEMP3Shield.h library, it would be enough to put different values for L and R, say, taking it from the “Trigger” example:

MP3player.setVolume(5,30);

For some reason volume is kept the same in both channels.Why is this? Is it a particular behaviour since I am using headphones?

Should I use separated speakers (LEFT SPEAKER/RIGHT SPEAKER) directly from their terminals in this Lilypad board, if I want to modify the volume per channel? Or is it that I am just not seeing something really obvious within the code?

Thanks, the sooner a solution appears, the better!