It sounds like the speakers are similar to ones which are supplied with a lot of midi systems. They are a sealed unit which is glued together during production. Although there is nothing wrong with this as such it does mean that there is no way in to change the cables.
I'm not an expert but I guess the best thing you could do is extend the cables. Either by cutting close to the housing (leaving enough to work with) or by adding extra cable to the existing end. As the cable is not carrying feed you could do this with anything from twisting the wires together and wrapping in serotype (I've seen this done when I was a student) to some sort of screwed or soldered connection system. Of course the sound quality may suffer slightly but I guess the speakers are not the highest quality anyway so the difference is likely to be unperceivable to all but top audiophiles (and they would probably recommend new speakers).
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