- Create a new SoundClass
- Open every Audio Content that should be affected
3. Set the SoundClass of all your selected Content to the Class we created
4. Create two new SoundMix Assets
5. Open both SoundMix Assets
6. Add an element to SoundClassEffects, set SoundClass to the Class we created and set the VolumeAdjusters to 0 and 1
7. Use the SetBaseSoundMix-Node (set to our new SoundMix-Assets) to modify our SoundClass
I hope this might help

For more informations check this: [Unreal Engine 4 Documentationt: Audio & Sound][6]