> 3. Is this a horrible idea and I am looking to cause a fire?
Depending on the topology of the PSU, simply soldering all the same voltages together alone is likely inviting a fire.
You're better off simply pulling 5 and 12 volts from the PC in your cabinet. Any power supply from the last decade or so can give up to 150 watts to a video card through the 6 and 8 pin connectors, which oughta handle your audio fine.