Constant Static/Hissing from Headphones

I've hooked my iPod up to my PC tower, and am listening to songs via iTunes. I have headphones plugged into the front audio jack (the only headphones jack--the others are for speakers/microphones). When it's plugged all the way in, The audio volume is cut in half, the vocal track almost disappears, and the whole song sounds like it's underwater or at a really low recording quality. Then when I pull the jack out a little bit, the volume is normal, but there's constant static/hissing. Depending on which way I turn the headphones, the static is in one or both of the headphones.

I never had this problem until yesterday, which is frustrating. I also encountered some weird things. For one, the static/hissing stops when iTunes is the active window, and I either use the mouse wheel or left click and hold on a song title/artist/album, etc. in the list of songs on my iPod. Right clicking does not make the static go away. But the static comes back if I move the mouse after clicking and holding it. I tried this same thing in Windows Media Player, and clicking the mouse buttons on any part of the active window won't do anything.

Things I've already tried:

• Trying the headphones (the earbuds that come with the iPhone) in different places--they work fine when plugged directly into the laptop
  
• Switching the PC power plug to its own outlet, instead of on a power strip with all the other computer stuff (the static seemed a bit better at first, but then it returned to normal)
• Downloading the latest drivers for my sound card (SoundMAX Digital Audio)
  
• Shutting off my cell phone to reduce interference (didn't do anything)
• Changing around the USB plugs so the iPod plug was far away from the mouse plug (didn't do anything)

So I was hoping someone could let me know what to fiddle with in order to get this annoying static to go away. I share an office with someone else, and I'd like to be able to listen to my own music quietly.

My machine is a Dell Optiplex GX620 with the SoundMAX Integrated Digital Audio sound card. Using a USB mouse and keyboard and the iPod USB adapter in the rear USB ports. Let me know if you need any more info to pinpoint the problem!

 

Not an expert, but if clicking on a song/changing the active window changes the problem, I would think it was a software issue... do you have a problem in other programs? Like WMP or RealPlayer?

 

The static starts in the headphones the instant they're plugged in, no matter what programs are running. But the only method of making the static go away is clicking the mouse/mouse wheel in the iTunes window or plugging the headphones all the way into the jack (thereby reducing the sound volume and quality by half).