dell studio 1555, windows 7, speaker fill does not work

hi mates.

I have dell studio 1555, upgraded vista to windows 7 64bit. I have properly configured my speakers to 5.1 surround setup in both IDT control panel and windows 7.

However, when playing from stereo sources (aka mp3, youtube), I only hear from 2 speakers only, even though I have selected speaker fill in windows 7 audio panel.

I have upgraded IDT driver to the latest one already, issue is not resolved

I would like to know how to make all my speakers play from stereo sources ( i know that this is not true surround)

Cheers