"The Lost Chord" is a song composed by Arthur Sullivan in 1877 at the bedside of his brother Fred during Fred's last illness. The manuscript is dated 13 January 1877; Fred Sullivan died five days later. The lyric was written as a poem by Adelaide Anne Procter called "A Lost Chord", published in 1858 in The English Woman's Journal.

The song was immediately successful and became particularly associated with American contralto Antoinette Sterling, with Sullivan's close friend and mistress, Fanny Ronalds, and with British contralto Clara Butt. Sullivan was proud of the song and later noted: "I have composed much music since then, but have never written a second Lost Chord."Many singers have recorded the song, including Enrico Caruso, who sang it at the Metropolitan Opera House on 29 April 1912 at a benefit concert for families of victims of the Titanic disaster. The piece has endured as one of Sullivan's best-known songs, and the setting is still performed today.

Sound recording and reproduction is an electrical, mechanical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording.

Acoustic analog recording is achieved by a microphone diaphragm that senses changes in atmospheric pressure caused by acoustic sound waves and records them as a mechanical representation of the sound waves on a medium such as a phonograph record (in which a stylus cuts grooves on a record). In magnetic tape recording, the sound waves vibrate the microphone diaphragm and are converted into a varying electric current, which is then converted to a varying magnetic field by an electromagnet, which makes a representation of the sound as magnetized areas on a plastic tape with a magnetic coating on it. Analog sound reproduction is the reverse process, with a bigger loudspeaker diaphragm causing changes to atmospheric pressure to form acoustic sound waves.

Digital recording and reproduction converts the analog sound signal picked up by the microphone to a digital form by the process of sampling. This lets the audio data be stored and transmitted by a wider variety of media. Digital recording stores audio as a series of binary numbers (zeros and ones) representing samples of the amplitude of the audio signal at equal time intervals, at a sample rate high enough to convey all sounds capable of being heard. A digital audio signal must be reconverted to analog form during playback before it is amplified and connected to a loudspeaker to produce sound.

Prior to the development of sound recording, there were mechanical systems, such as wind-up music boxes and, later, player pianos, for encoding and reproducing instrumental music.