I baked a sound as F curve (shape key) and when it ends…the influence of the shapekey is still 28% and not 0%…
but i want it to be 0% influence… how do i fix that?
Move last keyframe of fcurve down to zero position, maybe.
f curve doesnt have keyframes it says 
i can only use modifieres…but they dont work…
BUT i got it…i have to edit the sound in audacity…so it is zero at the end 
An excellent and pragmatic solution … far better, IMHO, than attempting to deal with it within Blender.
In many, many situations – computer graphics or otherwise – sound clips carry "noise’ that needs to be removed and clamped to zero before anything else is done with them. (The classic mixing filter that is used is called, appropriately enough, a “noise gate.”) This should be done in the upstream sound-editing process, so that every clip begins and ends with “zero = silence.”
Note that a classic noise-gate will clamp the sound to total-silence when the signal falls below its threshhold … but it will not truncate it. Audacity, of course, is an excellent piece of software which will do the right thing. (But, attempts to do the same thing directly in Blender might not, and discerning listeners will be rewarded with an audible “click.”)