Hi,
Thank you for the code.
What version of FMOD are you using?
I will link this forum which has discussed my solutions for VOIP issues: Vivox to Unity OnAudioFilterRead to FMOD programmer sound. Stutters/crackling - #15 by dougmolina
Sounds like an issue related to the buffer size, an option may be increasing buffer: FMOD Engine | Core Api System - System::Setdspbuffersize.
Hope this helps!