This has been moved to the Engine.ini file as of 4.6:
GConfig->GetFloat(TEXT("Audio"), TEXT("UnfocusedVolumeMultiplier"), UnfocusedVolumeMultiplier, GEngineIni);
In the DefaultEngine.ini or any of the Engine config files where you would need to change this just set:
[Audio]
UnfocusedVolumeMultiplier=1.0