actually as a thought I was going to say that you could actually use the jump to action part to create a loop & then have it gradually reduce the volume of the sound by x but having just checked the change settings of a sound action part I noticed there is no option to affect sound by a multiplier value, only the sliders exist..
I'm not sure if I get you right here, but I actually implemented a fading out like that. See screenshot. I connected a value "fader" to the decrease of volume and the speed is defined by the pause inbetween.