In v3.0.2 there’s a dialog box for audio controls:
This references a deprecated player, Sonic, which isn’t the default audio player. For almost all users, this option will be greyed out and unusable - and is just visual noise.
Mono audio is also functionality offered by the standard Android accessibility settings, which offer mono audio output for all apps. It’s in “Settings > Audio”. This setting in AntennaPod is a duplicate of an OS setting, which seems the wrong thing to do. Additionally, the OS setting works on every player. It appears to have been in Android since 2013, and there are copious mentions of it from 2017.
My suggestion is to remove this setting, to make the UX cleaner.
If it’s important to keep, a shortcut to the accessibility settings would make this feature available to every user, not just ones prepared to use a deprecated audio player.