This dissertation establishes the acoustic nature of three classes of soft polymeric materials: (i) thiol-ene linear elastomers with poly(dimethylsiloxane) (PDMS) network strands, (ii) brush- and comb-like elastomers with PDMS sidechains and n-butyl acrylate spacers, and (iii) ABA-triblock plastomer copolymers with linear poly(methyl methacrylate) (PMMA) A-blocks and brush PDMS B-blocks. Specifically, the acoustic attenuation, alpha, and the longitudinal speed of sound, Cnu, are determined as functions of Young's modulus, E 0, by applying pulse-echo methods to obtain ultrasonic time-of-flight measurements. With corresponding mechanical analysis, it is shown that these tissue-soft but solvent-free materials inhabit a unique region in the acoustomechanical space of alpha and E0, with alpha ranging from 1.2 to 5.9 dB/cm at 1 MHz and E 0 ranging from 2.9 to 185 kPa. Finally, it is shown with the plastomer and thiol-ene systems that the sub-atmospheric moduli additionally permit the complete in situ expansion of thermoexpandable fillers -- and that this in turn enables precise control of composite modulus, attenuation, speed of sound, and volume. Finally, it is demonstrated that these composites (i) expand with millimeter resolution under high-intensity focused ultrasound (HIFU) and (ii) yield considerable contrast in ultrasound imaging.