The correct spelling of the phrase "let the sun shine in" is often misspelled as "letted the sun shine in". The correct pronunciation of the word "let" has the "t" sound at the end, which may lead some people to mistakenly add an extra "t" to turn it into "letted". The correct phonetic transcription for "let" is /lɛt/, which indicates the absence of the "t" sound at the end. Therefore, the correct spelling of the phrase should be "let the sun shine in".