A classic case of sending unbalanced stereo into an input intended for balanced mono. 3.5mm is almost always unbalanced stereo (TRS or TRRS, tip and ring are right and left and I think the sleeve is ground). The inputs on pretty much any sound board has instrument in for right and instrument in for left. You're probably using a regular 1/4" to 3.5mm adapter to plug the computer in at just the right or left input on the board. A short bit about unbalanced vs balanced: so when you send an audio signal by itself across a cord that's really long, it is susceptible to rf interference. Radio waves in the air can interfere with it and distort it. What balanced audio does is it send two identical signals across 2 wires. It compares the two signals at the end and removes anything in the signal that isn't shared by both. Any interference experienced by one cord won't be experienced by the other cord the same way. So by removing anything that isn't identical in both cords, you remove anything that is interference. That's balanced audio in a nutshell. [EDIT: Strike that, reverse it.]
Now let's talk about how music engineers mix music when it is being produced. A lot of sounds they want to pan left or right to make the music interesting. You might want the bass sort of on the right and you may want the drums a little on the left. You may want piano a little in the middle and the guitar to go back and forth between ears. That's a really exaggerated example. But the point is that the signal sent to the right ear is intentionally different from the signal sent to the left ear.
When your audio board receives a signal in that left or right mono input, if there is signal both on the tip and on the ring, it expects both signals to be identical and anything that isn't exactly identical will be removed. The board thinks anything that isn't identical between ears is RF interference.
The audio sounds weird because your audio board thinks that anything in the mix that isn't panned directly in the center must be rf interference. It removes what it thinks is rf interference. So it removes a huge portion of your music. It makes your music sound super strange.
The solution is to get a cord that turns the unbalanced trs into unbalanced, separate, 1/4" ts cords.