physical velocity becomes to 0 and then become larger

I wrote a sound component that simulates the collision between physics and sounds out. Now want to become smaller as the relative speed becomes smaller. Material of the restitution is set to 0.6, but found that the speed is always the first small becomes to 0 and then become larger, and the actual phenomenon does not match, do not know how the matter, we have to know the reason? Thank you.
My speed function is used:BodyInstance-> GetUnrealWorldVelocity ();