Are you sure the 3.5mm input on the computer is a balanced input or for that matter even a TRS input to begin with?
It's not.
But wiring mono balanced audio + to tip (L ch) and - to ring (R ch) results in very close to perfect cancellation if anything is summed to mono in the PC or anywhere after that point.
I see it the other way around way too often.
L & R unbalanced audio from a PC feeding a single mono balanced input. Result is the difference signal (L-R) in which most bass and any mono vocals are cancelled out leaving reverb and swishy highs. (the root method behind many analog "Vocal Eliminators" used to generate karaoke tracks)
The quickest solution when you are in a rush is to just find a balance control somewhere in the PC and slide it to one side or the other. Just then hope the music being played isn't some old Beatles or AC/DC tracks!