Possibly pulse width modulation to control LED brightness? As I understand it, LEDs are dimmed by switching them on and off very rapidly, presumably at the frequency you're hearing.
Computer audio is prone to suckage. My PC's front (but not rear) headphone jack picks up interference from either the CPU or GPU (mostly heard during scrolling, so not sure which it is), as did one of my old Macs, and my BBC Micro, both of which played the CPU activity out over the speaker. My laptop has the same fault.
I reinstalled a Dell Latitude earlier that loses the attack phase of any sound that's played, and you can get clicks as laptop sound hardware is powered back up to play audio … And even modern PCs can skip audio under load, such as logging in :-P (Like how Windows 7's log in/out fade displays tearing on most computers, which is truly pathetic.)
(My 486 had a really nice C-Media sound card where there were no audio failures, just that the headphone jack came apart and came loose from the PCB ;-) I've got a SB16 in there now and either the General MIDI patch set or the OPL3 circuitry is nowhere near as good as the C-Media CMI8330, and the drivers are a memory hog and broken. C-Media managed to write drivers that consumed no memory at all, they just seemed to configure the card and exit. C-Media were the Chicony of sound cards – good quality wrapped up in flimsy.)
Sound sucks :P