@quan chi, u can use them as 4.1...
Set quadraphonic speakers in your audio control panel, connect the 2.1 as front & 2.0 as rear.
Select full range/large speakers(name differs in each audio control panel) so that the 2.1 distinguishes the bass signals by itself...
I guess ur audio I/O conn. looks like this...
Are u saying that while playing a video, only 2.1 works?