Final answer:
The Beatles song that marked the first use of reversed recorded sounds in a pop song is Strawberry Fields Forever.
Step-by-step explanation:
The Beatles song that marked the first time that reversed recorded sounds had appeared in a pop song is Strawberry Fields Forever. In this song, the Beatles used a technique known as tape looping to create a mesmerizing effect.
They recorded certain passages of the song, played them backwards, and then incorporated those reversed sounds into the final mix. This innovation was highly influential and contributed to the experimentation and innovation within popular music during the late 1960s.