1. RCA Stereo Y Adapter, plug the small end into the sound card, plug the big end into the head unit's auxilary input, or an amp. Then if you have an amp, run normal speaker wires from the amp to the speakers.
2. Well you have to hook the caputer on your home network, wirelessly (wireless is very helpful on a caputer) or if you have a usb flash drive u can use that, but its about the same price for a wireless adapter.
3. Do a search on xtreme-radio, and D-Link Radio