if you have the correct drivers, and you only hear out of one speaker, and you know the balance is 50/50 - it has to be hardware and the speaker must be replaced. To know for sure you need to get to the 2 wires connected to it. A simple test is to get a cheap set of earbuds you are not using and cut off the 3.5mm connector and strip the wires to one earbud. With it connected to an mp3 player of your choice, connet the stripped wires to the speaker - if it works, you know its the software.
this is not a problem with the drivers, but most probably a bad connection to the computer. Another thing to check is in the sound mixer, make sure that the balance slider above the volume control is in the middle if you don't have this or you are running Windows 7 you can find the balance control in "control panel,sounds,playback,speakers,properties,levels,balance"