I've owned these for about 4 years:
http://www.amazon.com/Logitech-Z-640-Speaker-Surround-System/dp/B00006HYPD

1) What is that big fat square block for?

2) Also, when I play my 360 with this I have a black cable and a green cable that I plug into a red/white adapter. The sound, however, works even if I only have the green cable plugged in. What does the black cable do?

The big fat square block is the subwoofer. It's used to deliver all low tones.
It usually isn't counted as a full speaker, that's why it's called 5.1 sound and not 6 sound.
You can also put it more or less anywhere in your room, because it's harder to localize the source of low sounds.

Putting right under your chair is ... ummm... entertaining?

And sexy!

man.

when will computer oriented speakers have a decent midrange reproduction?! screw all that countour, gimme my 500khz!

it has to be asked.

If you've owned them for four years... Why on earth didn't you just RTFM? Or just plug it into the rest of the system? Hell, it came with it all. If it came with it all, wouldn't logic conclude that it's NOT going to blow something up, set fire to something, or collapse the universe into a tiny ball of gas from which there was never any hope of escape?

Never heard of a subwoofer?

Personally, I don't often hear much of subwoofers. Maybe I'm buying cheap brands.

I did always plug in both cables, until noticed zero different if the black cable was not plugged in. So I'm wondering what it does.

My speakers cost 3 bucks at a garage sale!
Also they are the best speakers i have ever seen!
That is all.

I take it you haven't seen many speakers.

There's a volume adjust on the back side. Hell... even with that on the lowest setting you should've heard SOMETHING though

Your bass setting is probably not turned up on the speaker or on your sound card, or both. If it were working, you'd clearly hear bass guitar and bass drums in any music you listen to.

Tiberium's "poor me I don't have a good computer" post #2343.

Let me guess. Tin cans and string? You got ripped off.

Not necessarily. People sell old, high quality stereo equipment all the time for very little, in favor of some new shiny thing with "digital" written all over it. That being said, he still doesn't know what he's talking about.

Well, I still use a pair of Yamaha speakers from a computer my family bought back in 95. But atleast I know they aren't the best quality. They work fine though.