Older Harman Kardon computer speakers have loud hum

Replace your speakers as one is going bad

You may need to have it repaired. Have you tried a Universal remote control , this may work out a lot cheaper. Hope this helps

Line is grounded, check the wire connector there might have been a short on it!

The hum (50 or 60 Hz) is caused by the AC power getting into the entry point of the amplifier. Either the filtering capacitors from the powersource of the speakers have dried up - replace them - or there is a short in the cables.
There is another possibility: if you have kicked the subwoofer  box its transformer might have slipped from its fixings and now is touching the subwoofer speaker frame.
The only way to find which is to open the cases.

If it's a loud 60 cycle hum stop running it and have it serviced. This hum can be a shorted output which can send out 50 to 60 volts out to your speakers damaging them.

Check for a Fuse. Mine is an older model and has a Fuse right next to the power cord. It probably has a slot for a flathead screwdriver.

Unplug the minijack from computer to hear if it still hums. If not test speakers at another computer. if it works fine on other computer the sounncard is generating hum noise. Common problem is usually the earthing cable inside the minjack that is touching L or R speakerwire due to bending of cable and breaking off insulation. If youre up to it cut off minijack if its moulde and resolder a new one on.

HI
Time to change sub, or RMS product, for a change, if warranty.
it is a problem with the resistor inside sub.

speaker amplifier chip may leaky or damage
you are using improper sound driver
check driver status in your computer device manager or just typing dxdiag on run in start menu, check status of sound driver installation

have you plugged anything else in to make sure the usb port is working? does the computer recognize the usb speakers when you plug the usb plug into the computer? do the speakers work on another computer? if you wiggle the usb wires around does it make a sound through the speakers? eliminate these questions so that we can move on to isolating the issue.