So I moved into a new house where the previous owner put speaker wire behind the floorboard for rear speakers in the family room. I have a "computer" 5.1 surround system that uses RCA cables to ...

bad idea, RCA uses an impedance of 75 Ohms, which you fuck up when you splice the wire. I'd recommend using RG6 cabling for the video, and, if possible, S/PDIF for the audio, but in lieu of that I ...