A wearable infusion pump assembly including a reservoir for receiving an infusible fluid, and a fluid delivery system configured to deliver the infusible fluid from the reservoir to an external infusion set. The fluid delivery system includes a volume sensor assembly configured to receive a quantity of the infusible fluid from the reservoir. The volume sensor assembly includes an acoustically contiguous region having a volume that varies based upon the quantity of infusible fluid received from the reservoir, and an acoustic energy emitter configured to provide acoustic energy at a plurality of frequencies to excite a gas included within the acoustically contiguous region.