I’ve tried muting sounds with SoundClass->Properties.Volume = 0.f; and pushing a sound mixer that sets volume to 0 on the Master sound class, and it works fine on PC and iOS, but not on Android (Galaxy S4, Galaxy Tab, etc).
I can still hear the sounds if I put the device sound settings at maximum volume.
Take a look at this post that is dealing with a similar issue. You could be calling the incorrect SoundClass to mute. Try editing the MasterSound class which controls all of the volumes instead of the SoundMixModifier.
How to Change Volume C++
Let me know if this post had the correct solution.