I have two sets of Logitech Z313 (as well as another pair of speakers from before). Each Z313 has a sub-woofer and two speakers. Is there a way I can set these up to make a 5.1 system or something similar?
From what I can tell, the z313 just connects to the main audio out port on a PC, and the subwoofer signal is derived from what it gets there.
If you are using this with a desktop that has 5.1 ports or more, ie one port for front speakers (green), one port for rear speakers (black), and one port for centre/subwoofer (orange), then yes.
You can connect the second set of speakers to the rear speaker port, usually coloured black.
Then you just need to go to the sound settings in Windows and let it know you have rear speakers.