Hi,
After some investigation, it seems that this issue is linked to a previous forum post here: Volume difference between Audio APIs - #2 by jeff_fmod.
Unfortunately, we are not able to solve the issue at this time. A workaround may be changing the output used by the FMOD core system to OpenSL.
I would recommend using the Unity Integration | Scripting Examples - Callback Handler to change the output before the FMOD system initializes. To change the output you need to call FMOD Engine | Core API Reference - System::setOutput.
Hope this helps.