There isn’t a way to get the real-world volume of audio yet in UE4. As in, the volume of the waveforms playing for a given voice. You can get (what I call) the logical volume of a wave instance using GetVolume() (at head revision) or GetActualVolume() (at earlier revisions). But that’s not exposed to BP.
However, I think you should be able to get what it sounds like you want doing OnAudioFinished delegates, which triggers a BP delegate/event when a sound on an audio component finishes playing.