Disclosed are pump-down sensor devices that operate in conjunction with a fiber-optic sensing system to take downhole measurements and communicate them to the surface while moving untethered through a borehole. A pump-down sensor device in accordance with various embodiments includes one or more flow baffles configured for a specified buoyancy, and an electronics module for measuring one or more downhole parameters and transmitting an acoustic signal encoding the measured parameter(s). The acoustic signal can be detected using the fiber-optic sensing system. Additional embodiments are disclosed.