From Wikipedia: I am sitting in a room is a sound art piece composed in 1969 and one of composer Alvin Lucier's best-known works.
The piece features Lucier recording himself narrating a text, and then playing the tape recording back into the room, re-recording it. The new recording is then played back and re-recorded, and this process is repeated. Since all rooms have characteristic resonance or formant frequencies (e.g. different between a large hall and a small room), the effect is that certain frequencies are emphasized as they resonate in the room, until eventually, the words become unintelligible, replaced by the pure resonant harmonies and tones of the room itself.