Wikipedia on composer Alvin Lucier's I am sitting in a room (1969):
I am sitting in a room (1969) is one of composer Alvin Lucier's best known sound art 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.
In his book on the origins of minimalism, Edward Strickland wrote that "In its repetition and limited means, I am sitting in a room ranks with the finest achievements of Minimal tape music. Furthermore, in its ambient conversion of speech modules into drone frequencies, it unites the two principal structural components of Minimal music in general."