For General Reference:
Grounding (bonding) of all units.
Both home computers and home stereo can be damaged by wiring mishaps. The output of a PC sound card is at a low level. Nevertheless, both the PC and the Stereo can be damaged by careless use of audio connectors.
I used to do professional level audio work. Whenever you have disparate (read portable) equipment being hooked together, you must have a grounding strap to all equipment before you do anything. Just because you got away with it before does not prove the equipment has auto protection for ground currents.
In professional work things are put in large racks that tie all snuff together and disperses the ground currents. In amateur work only a few audiophiles understand there has to be a ground bar installed first.
Translation. run a small wire to the frame of each piece of equipment before you turn the power on.
Ground loops are a source of niche in audio systems. But they can do damage in extreme cases.
https://en.wikipedia.org/wiki/Ground_loop_%28electricity%29Ground loops are a major cause of noise, hum, and interference in audio, video, and computer systems. They do not in themselves create an electric shock hazard, the inappropriate connections that cause a ground often result in poor electrical bonding, which is explicitly required by safety regulations in certain circumstances. In any case the voltage difference between the ground terminals of each item of equipment is small. A severe risk of electric shock occurs when equipment grounds are improperly removed in an attempt to cure the problems thought to be caused by ground loops.
In the USA and many other places there is supposed to be a safety ground wire on every electrical outlet.
But sometimes it is not present. Then you can get a significant shock. You can survive, but audio input devices for home use can not tolerate that kind of abuse.
The input limits for such devices may be as low as 5 volts and 20 mills.
Output devices have a higher limit, but still much less that the 120 VAC That might be present with no grounding in place.
So the OP may have blow stuff in the sound card. And even the stereo.
Just for informational use.
I really do not know what actually happened.