Polyethylene is a widely used plastic exhibiting a large range of properties that depend on molecular weight, crystallinity, chain branching, and cross-linking. In this study, the sound speeds and elastic properties of a variety of commercially available polyethylene materials were experimentally determined using the pulse-echo ultrasound technique. In situ pressure dependent measurements, including ultrasound time of flight, x-ray diffraction, and x-ray radiography, were performed using a Paris-Edinburgh large volume press at the High Pressure Collaborative Access Team (HPCAT), beamline 16-BM-B at the Advanced Photon Source. Polyethylene sound speed and elastic moduli were found to increase with increasing pressure. The zero pressure orthorhombic phase was found to transform to monoclinic at low pressures of ~0.1 GPa. Display omitted •Sound speed and elastic constants in polyethylene increase with increasing density.•Sound speed sharply increases at low pressure due to compression of free volume.•Stress-induced martensitic phase transformation observed at very low pressure.