If an audio source is mixed to already be close to the maximum possible loudness, it can cause an overloaded input into FxSound, and produce distortion and intermittent ducking.
If this happens, reduce the source’s volume settings by around 20-25%. Use FxSound’s Dynamic Boost and your computer’s overall volume to compensate for the reduced amplitude.
FxSound also uses its own virtual surround processing that can sometimes conflict with other virtual sound processing. When using FxSound, you will want to make sure that you have virtual surround settings turned off for all of your audio devices.