I'm using the Realtek HD audio and today for no apparent reason my Logitech X-240 speakers have a noticeable hiss even at lower volumes.

It was not like this earlier this morning but when I came home tonight and sat down, there they were, hissing away. I've already tried muting all inputs and outputs but none have any effect at all.

They don't hiss when the sound wire is NOT plugged in.

They still hiss even in front headphone port.

They DONT hiss when plugged into my psp.

How can I solve this problem?

Try updating the drivers.

Also, what motherboard you using?

<puts on audio hat>

Hissing in speakers is generally due to noise in the system somewhere getting amplified. There will always be a certain amount of noise, but what you can do is check to be sure your input gain is set on the high side. That helps ensure there's maximum signal to the (sadly, generally fixed) amount of background noise. Then adjust overall volume from the master.

The other thing is to try and isolate the cause of the noise by switching around between various end sources. So - If the noise is tehre when the DVD player is on, but not when it's off, then you know it's either the player or the interconnects to/from the player. Same on a computer - Try to mute and unmute various sources.

try muting your line in and any else that is not in use in the sound options

makes a big difference

Onboard audio is typically not high quality, it's possibly your problem, it's also possible you've got a component in your system that's injecting more noise than usual (maybe something that's close to failing... or that isn't intended for consumer use). I have Realtek onboard audio on my HTPC but I replaced it with an Audigy card a couple years back, using the optical output to my audio rack keeps everything digital in the PC where you're most likely to get noise.