I've got a Soundblaster Z with zero problems at all, the fact that it is related to a USB mouse and your HDD pretty much just indicates you have a problem that is revealing itself and allowing you to know it exists, through the soundcard.
I agree with the already mentioned ground, something is not grounded that is common to the USB and HDD circuit.
So you need to diagnose the problem and start by some simple process of elimination.
These are the simple things to test, and do not discount these tests until you confirm each is problem free.
You could have a bad mouse as far as wiring goes in the connection, I suggest swapping out a known good mouse and see if the problem persists?
Use a different USB port for the mouse and make sure you use the proper procedures for relocating the mouse don't just pull it without the operating systems OK?
If those tests are no joy:
Shut the system down and pull the speaker cables and completely pull the soundcard, then reseat it back in the machine, reconnect the cables and test?
If the problem persists follow the above with the soundcard but this time relocate the soundcard to another slot, even if it is a PCI-E sound card and you don't have another smaller PCI-E slot you can use a vacant full size PCI-E slot to test in, if you have one?
Check all your cabling in the machine and make sure everything is fully seated, HDD cabling as well?
Well if none of the above has resolved the issue, I'm sorry but the easy stuff is over!
It's time to pull the motherboard!
Most common issues with this situation is either a motherboard mounting plate stud support was left in that should have been removed as there should only be a stud located for each M/B screw hole, I would inspect for that first which means pulling the motherboard, as my next suggestion is to double check the tightness of the motherboard mounting screws. (However if there is a mounting stud under the M/B that is not supposed to be there, continued tightening down on it could short out the M/B!)
Also inspect and make 100% sure the I/O plate is not somehow shorting out if it is a metal finger ground type?
If you run all these tests and you still have the problem RMA the motherboard!
You should never be able to hear Mouse and HDD functions through your discrete soundcard, I don't care what brand it is.