Physics? We don't need no stinkin physics..
EE, different story. gotta have EE's.
The ground wire of a line cord can carry the signal return current. Not the IC shield.
Think of it this way...use a twisted pair in place of the IC. It will probably be good enough. Now, untwist this pair, and route the ground wire on the line cord to the wall outlet, then tape it to the source line cord to the source output jack. You have formed a huge loop which will trap external magnetic fields.
That is what most audio systems do.
Changing the PC can indeed alter a system, specifically if that system does not control the input signal current properly. Single ended drive is an especially large offender in that regard.
Any PC that changes the sound of the system does so because of the system, not because of any material or silly physics property of the cord.
jn