Thanks for the quick reply Suman,
Although I tried what you mentioned previous to you posting it here, it didn't solve anything. It's as if only one slider was working, causing the audio to change from right to left when turning it down.
What I did manage to narrow it down to was indeed my soundcard / drivers, and not Windows itself. I found on another page that somebody else had the same problem, but instead of using the ASUS drivers, they used third-party drivers that are meant for the hardware.
I don't know if I can post the link here, but here is the thread that I followed to get it fixed:
https://www.infopackets.com/news/10456/how-fix-asus-xonar-wont-play-51-surround-sound-win10(With bonus options for 5.1 surround sound, which was also not working on the ASUS drivers.)
For anyone else with this type of card, I recommend the Uni Xonar Drivers.
*Edit: They work just as good if not better than the original drivers. I have no issues so far, and the balance problem has gone away.