the Z-5500's use 3 x 3.5mm audio cables (for 5.1),
or an optical cable,
or a single 3.5mm audio cable at the side (for stereo, which they can remap to all channels using the 'stereo 1.3' switch at the back)
It's possible to just use a different connector for each system (eg the 3 x 3.5mm audio cables for the PC, the optical cable for the xbox), and then switch between them by pressing the 'input' button