Have you tried to isolate where the problem is?  e.g. is it the speakers or something else? 

Connect headphones to the front panel headphone jack and see if the sound is "bad" there.  If not then it may be the speakers.  The first thing to do is "reseat" each speaker connector - the one to the PC and the internal connections - reseat is to unplug each plug then reinsert it to the connector making sure it's fully seated.  If that doesn't fix the problem, plug the speakers into a portable CD or Tape or evena an MP3 player and see if the sound is bad there too.  If it is then that will definately prove it's in the speakers and as they are basically "non repairable" you will have to buy a new set of powered 2.1 computer speakers. 

If the headphone output is also "bad", check to make sure all the volume controls for each sound type and the master are not set low (the sliders should be up or at least mid way).  If your system allows you to adjust the bass and treble make sure they are set mid way.  Some media players also have EQ settings and some even have some addtional settings - make sure that is not causing it.