Hi,
Thank you for all the information.
What version of the FMOD integration are you using?
Would it be possible to have a look at both of your implementations to see if there is any way I can help?
A user has implemented something similar here: Vivox to Unity OnAudioFilterRead to FMOD programmer sound. Stutters/crackling - #15 by dougmolina. It is using C#, however, it may have some useful insights.
Unfortunately, I think you have touched on both methods.