1:
Did you set your audio driver to output 2.1 instead of 5.1?
Because when u set to 5.1, only when there is more
bass(like explosion in a movie), the bass is then passed via
another 'output jack' to the subwoofer.
If you listen to a song while in 5.1 setting, the bass is
usually attenuated and only the front two speakers produces
sound.
But you are using 2.1, so there isnt another wire for the
subwoofer that connects to your computer
I hope im right, cos im too poor to experiment with 5.1
2:
Did you turn the 'volume' knob for the bass up?(If any)
3:
Loosely inserted audio cable will also caused the same
problem
4:
Wrongly inserted audio cable(plugging the cable into the
microphone port, and selecting that port as the audio output, will
cause a little decrease in quality)