While I might be wrong here, I've noticed that the volume sliders are exponential when using after a certain version of renpy (likely 8+ or whenever 'get_mixer' was introduced to replace get_volume, and since it's the one I experienced this on). Replacing get_volume with get_mixer seems to fix the quirk, but nonetheless I thought useful to give a note in the issues for the template.

For reference, I found this renpy post -> https://lemmasoft.renai.us/forums/viewtopic.php?t=67398