Although the Apple IIgs has stereo capabilities, for some reason, Apple decided to neuter this feature on the rear output jack. The issue has been resolved by Bob Darlingtons LiteSound Stereo Audio Interface card. Here's the install and review.

To purchase the card - https://jcm-1.com/product/litesound-a...
Bob Darlingtons Youtube page -    / @bobdarlington  
Bob Darlington’s Github Page - https://github.com/BobDarlington/Lite...