Speed of controller trigger pull

I’m working on a VR project with the Vive.

When I squeeze the controller triggers, the VR Pawn’s hands animate from open to close, and it varies with the speed at which I pull the trigger. There is a direct correlation between the amount of trigger pull and the amount of fist being shown (hope that makes sense).

I would like to track the speed at which the trigger is pulled so I can play different audio, based on the rate.

Any idea on how to accomplish this?