In the opening sequence, the family of the founding members of the Beach Boys (I forget the names) is recording a song in the garage on an open-reel Nagra reel-to-reel cassette recorder.

When the youngest son walks in, the rest of the family is overdubbing (recording over top what has already been recorded) a piano track. Then the youngest son his encouraged to sing over top the tape so they can have three-part harmony vocals. This was not possible during the 50's (the time period the first half of the movie took place in).

This is a technique known as multi-track recording, and it wasn't invented until the mid-60's.