Ultrasound can power implanted medical devices. This paper evaluates its feasibility for microscopic robots in tissue that mechanically harvest energy using pistons. At these sizes, viscous drag dominates the piston motion and acoustic attenuation limits how far power can reach. Combining these factors shows that frequencies around 100kHz can deliver hundreds of picowatts to well-separated micron-size robots in low-attenuation tissues within about 10cm of the skin. However, applications of microscopic robots could involve large numbers, in which case the robots themselves significantly increase acoustic attenuation. Robots can mitigate this attenuation using cooperative swarm behaviors, with trade-offs among individual power, group performance and the complexity of the robot controllers. With such mitigating behaviors, acoustic power can be useful for swarms of a few hundred billion robots in the body, that each use tens of picowatts, on average, and can tolerate significant variability in available power, e.g, as robots in the bloodstream move from near the skin to deep within the body, or from low- to high-attenuation tissue such as the lungs.