What kind of speakers do you have? It could be a few things, the PC putting out radio waves, unshielded crappy speakers, bad cables, audio settings, speakers too close to the computer and etc.
The fact that moving your mouse causes a noise change, I have heard that before.
In general more info needed on your speakers and computer.
Hey my speakers are Logitech Z130. It also comes through my headphones which are the Razer carcharias.
my computer is an alienware Aurora
Intel® Core™ i7 processors
Windows® 7 Home Premium, 64bit, English
16GB 1600Mhz Quad Channel memory*
4GB GDDR5 NVIDIA® GeForce® GTX 690
2TB RAID 1+0 performance hard drive