Mechanical properties of Arctic sea ice can be inferred by observations of in-ice propagation of compressional, shear, and flexural waves. During the 1980s, impulsive signals were generated by a lead ball or sledgehammer dropped onto the sea ice, and the inference required observation of wave speeds. During ICEX20 and ARCEX23, passive cryophone observations were made of naturally occurring compressional wave resonances. Average first-year ice thicknesses during ICEX20 and ARCEX23 were inferred to be 1.3 and 1.6 m, respectively; these are consistent with independent observations and indicate the potential for remote, autonomous monitoring of sea ice thickness.