It's not the PSU. The extra 4 wires carry a line each of 3.3, 5, and 12 volts and a ground line.
A G31 board doesn't pull much power. You don't really need the extra 4 pins. My Gigabyte G41M-ES2L with an E6500 Wolfdale OC'd to 3.6 GHz, 4 GB of RAM, and onboard graphics only pulls 110 watt from the wall as measured with my Kill-a-Watt meter.
Use an ohmmeter to measure continuity through the speaker and wiring. If it is bad or questionable, you can get a replacement here:http://www.cwc-group.com/casp.html