Like this works in C++:
bool USomeBlueprintFunctionLibrary::SetSoundClassVolume(USoundClass *SoundClass, float Volume)
{
if (!SoundClass)
return false;
SoundClass->Properties.Volume = Volume;
return true;
}
Current marketplace packages that tries to workaround that issue have to do weird stuff like creating many, many mixer presets (Generic Shooter Sample Project) or start to ignore sound-classes and lead users to pull wires with values to every sound in game (Quality Game Settings) - which would be very frustrating if project grows.
Further workaround - using plugin:
Issue was already discussed in the past:
https://answers.unrealengine.com/questions/133720/cant-manipulate-audio-volumes-from-blueprint.html
I think this would be very easy to implement and would make blueprints look a bit more complete.