Hi Geoff,
Your TauPlayer has its LoopMode parameter set to TimeIndex which means that the rate at which the Tau file is being played is controlled by the expression in the TimeIndex field. The TimeIndex field in this Sound is a function of !PenX, so left/right movement of the Wacom pen will control the rate.
In other words, the Rate field has no effect when LoopMode is set to TimeIndex.
If you would like to trigger the Sound with the Wacom and control the rate of the playback with !Rate, you could try changing LoopMode to Live.