During playback on Plex player on your Mac, go to https://app.plex.tv/desktop, click on Settings (near the upper right corner), then on the scrolldown to your Server, and then below that, click on Dashboard. Post a screenshot of the Now Playing section (expanded), like this:
Model Name: MacBook Pro
Model Identifier: MacBookPro12,1
Processor Name: Intel Core i5
Processor Speed: 2,9 GHz
Number of Processors: 1
Total Number of Cores: 2
L2 Cache (per Core): 256 KB
L3 Cache: 3 MB
Hyper-Threading Technology: Enabled
Memory: 8 GB
Boot ROM Version: 186.0.0.0.0
SMC Version (system): 2.28f7
I am using a RME Fireface sound card . Connected Dynaudio speakers and sometimes listening through various headphones.
I am in a sound studio so I have a lot of things happening but i am sure the mono issue is not hardware related. Everything else is stereo and the web player is stereo just when I play through the player it is mono