Possible to MOD a PSU for -5v?

I seem to have a problem. I purchased a Coolmax 700w PSU recently from ebay to replace a defective PSU in my system. Almost immediately I noticed a problem. Now my system has no sound. I checked all kinds of stuff. Thought I shorted something out. Nope..
Then I remembered that my system, MSI MS-9620 (K8N Master2-FAR) requires -5v for the sound chip to function. This PSU has no -5v. Is there a way to modify this PSU to produce the -5v that I require?
The PSU model is ZU-700B

Thanks

if you can find a small dc 5v power supply (eg cellphone charger or dc power supply for some other device) you can connect its +ve terminal to ground & use its -ve lead as minus 5 if its output is floating (ungrounded). if i was doing it i'd plug the dc adapter in & connect its +ve terminal to a ground via a resistor & check to see if there was a voltage across the resistor with a multimeter (no or little voltage means its suitable, 5v means its grounded).