Weird sound problem. Options

[grgspunk]

As mentioned in TLTE, I bought an audio cable yesterday connecting my PSP's headphone out with my laptop's Line In-socket. This works fine and produces an okay sound until I connect the PSP further via an USB cable (necessary for recording videos, of course). Suddenly, there is a certain latent noise humming in the background, which persists for as long as the USB cable is connected. If I pull it out, the sound returns to normal.
Even more bizarrely, if the mouse is plugged into the laptop at the same time the noise is louder, and turns into a loud beeping sound whenever the mouse is moved. The tone of this beep changes when it is lifted up.
What could the source of the problem here and how could I solve it? I assumed that the audio cable might merely be of a low quality and susceptible to electromagnetic disturbances, but that doesn't explain why the mouse has no effect on the sound unless the PSP is connected via USB.