How do FadeIn/FadeOut work internally? Can fades be arrested/reversed after being triggered?

Rather than using audio component fade in/out directly, you can just do the fade yourself in BP and set a volume value with a shorter fade duration in a BP timeline callback. You could also just hook it up to a UE4 curve asset and do your own sweeping through the curve to control the fade. You’d only then need to use an audio component fade time value equal to the the time-delta between updating your manually controlled fade curve.