For a start, the mic port is a connector to feed a signal In to the sound card. So plugging a speaker into it does not sound right. (If you will forgive the unintentional pun)
On my system the bass unit plugges into two spk output plugs, and the rest of the speakers plug into the back of the SUB, which is also the amp. The sub also has a mains power supply, the if not plugged in to the mains, or turned off stops the whole lot from working. There is also an on/off switch hidden on the back of the sub.
Hope this helps.