We present compact microwave-optical transducers between the microwave S-band and optical C-band based on monolithic integration of piezoelectric actuators atop silicon nitride photonic circuits. The silica cladding of the optical waveguide core is suspended below the actuators, forming a bulk acoustic resonator that mediates interaction between microwave and optical fields through piezoelectric and stress-optical effects. An off-chip photon conversion efficiency of −48 dB is measured over a bandwidth of 25 MHz for both up- and down-conversion. Microwave pulses are synthesized on-chip through down-conversion and multiplexed transduction is demonstrated, indicating its potential as a photonic interconnect between cryogenic circuits.