Ground Roll sounds

Wondering if anyone would know just how many GROUND_ROLL sounds are allowed for a sound.cfg file? I am trying to use three for an A320, all during different stages of the roll. The third one is the pronounced 'rattle' I always seem to hear in the cockpit about the time of Vr. I've sampled this, and set it to play between 120 and 150 knots (120 being set at volume 0, 150 at volume 10000). However, it does not sound when the speed approaches. Any ideas? Thanks,- daniel