When playing a sound cue inside a UMG widget blueprint using any of the Play Sound functions, the sound always plays at full volume, even if the sound volume is lowered inside the cue file (Volume Multiplier).
The only way to lower the sound is to actually lower the volume in the Sound Wave itself.
Not a perfect solution, but you can get your UMG widget to trigger an event in your character blueprint and have your character play the sound. This would let you carry out your work while this gets fixed I guess.
In 4.8 we’re deprecating that function, and it’s being replaced by a global function that’s just PlaySound2D that takes all the extra parameters (like volume adjustment) that PlaySoundAtLocation does (sans location).
While experimenting today with sound and UMG I have noticed that it’s not possible to stop a looping sound from playing (if played inside the widget class). The only way to stop a sound is when using a Sound Component. And Sound Components aren’t available in UMG widgets. Probably something to take into account with the new system, to allow stopping a sound in addition to playing it.