Cables are usually the first point of failure especially if you constantly plug and unplug them. Next would be the jacks themselves. Short of experiencing a power spike, also check your soundcard's mixer settings. Some software like media players and games trend to fudge around with the volume settings. Lastly, if you experience power spikes and or outages, the acrid smell of of burning silicon will tell you that a component has failed; that's why i often recommend getting a UPS for sensitive electronic gear.