Many third-party speakers, such as the ones made by Harman Kardon, come with a subwoofer and a set of speakers that typically play a better sound than the speakers that came with your PC, or the speakers built into another audio device.
Connect the speakers to the subwoofers using the 3.5mm stereo cable that came with your Harmon Kardon speakers. The cable and the proper jacks are usually colored the same, with the usual color being green or yellow.
Video of the Day
Connect the subwoofer's "Audio In" to the "Line out" or output jack of the audio device you are connecting the speakers to.
Connect the power cord to the subwoofer, then connect the cord to the AC power outlet.
Set the volume on the speaker to the lowest setting, then turn on your audio source.
Play something from the source, then raise the volume of your speakers until it is at a comfortable level.