PC speakers?

I have a Dell PC with iTunes that my son just started to use for playing music. The current speakers are Harmon Kardon with a sub. There is one wire that comes out of the sub to power the two samll cube desktop speakers (it splits into two wires) and one of the cubes does not work. What would be a better choice to work with the existing sub, not too large and can sound better then the cube speakers? One of the cube speakers has the volume and on/off control.

Thank you.