gainCurve is volume, pitchCurve speaks for itself
first value is gain/pitch, second is speed, you can work out the rest
I would try a gainCurve like this for your bell { 0.0, 0.0 },{ 0.0, 0.2 },{ 1.0, 0.21 },{ 1.0, 0.25 },{ 0.0, 0.26 }
this would make it play at 1.0 volume only when the speed is 0.21-0.25, play around with that