Organized assemblies of cells have demonstrated promise as bioinspired actuators and devices; still, the fabrication of such "biorobots" has predominantly relied on passive assembly methods that reduce design capabilities. To address this, we have developed a strategy for the rapid formation of functional biorobots composed of live cardiomyocytes. We employ tunable acoustic fields to facilitate the efficient aggregation of millions of cells into high-density macroscopic architectures with directed cell orientation and enhanced cell-cell interaction. These biorobots can perform actuation functions both through naturally occurring contraction-relaxation cycles and through external control with chemical and electrical stimuli. We demonstrate that these biorobots can be used to achieve controlled actuation of a soft skeleton and pumping of microparticles. The biocompatible acoustic assembly strategy described here should prove generally useful for cellular manipulation in the context of tissue engineering, soft robotics, and other applications.