The Sony PMC-MD55 is a digital voice recorder that allows users to capture and save audio recordings. It features a built-in microphone and supports external microphone input for higher audio quality. The device has a built-in speaker for playback and a headphone jack for private listening. The PMC-MD55 also has a voice memo search function, making it easy to locate specific recordings. Recording files can be saved directly to an SD card or transferred to a computer via USB. Some additional features of the Sony PMC-MD55 include adjustable recording volume, noise reduction, and auto-volume adjustment. It is a popular choice for professionals in various fields such as journalism, business, and education due to its high-quality audio performance, ease of use, and affordability.