I think GadgetSmith is correct.
From what I could find out, you should be able to use the standard headphone extension cables (
3.5mm Stereo Plug/Jack M/F Cable) found on monoprice. They come in 6', 12', 25', 50' and 75' lengths.
Here's a link to your
Logitech Z-5300 Installation Guide (PDF link found near the bottom).
On
this Logitech web page, you'll find a picture of the cable ends.
You might want a second opinion before buying the cables, though.