Set master volume in PIE

If you are developing a level and there's background audio or music that hits you in the face every time you start PIE, you can set a simple override for the audio volume (which you may need to do if you also want to play other audio sources from your PC and don't want to simply adjust the Windows volume slider).

In your player controller, or any blueprint that is exposed to the scene at the onset, add the following:

Create a Sound Mix and a Sound Class in your project folder. These are under RMB > Audio in the content browser asset types. The Mix is the one that is used to adjust the volume, and the Sound Class helps to categorize types of audio (ie, Music vs UI sfx, or Environment Ambient noise) ... anyways, set the Mix as the variable value of the In Sound Mix for the Set Base Sound Mix node in the blueprint.

In the case shown, you'll notice the In Sound Class being overridden is 'UDS_Weather'. This is the Sound Class of the plugin UltraDynamicSky (highly recommended). It has a very loud stormy wind sound. In my case, I want the windy sound in my game but I don't want it to blast me every time I test a level change. You can set the UDS Weather audio settings to be lower, but then you have to adjust them back again later on, so a quick override is handy. Finally, in the Set Sound Mix Class Override, just lower (or raise) the Volume value and it will moderate every sound asset that is managed by the assigned Sound Class. That's it.