Corresponding letters of the alphabet with 相当

Kanji Explanation: Mutuality and appearance with 相

Following a series of murders, Conan discovers a message hidden in sheet music. He explains it to Ran.

コナン:: 「ピアノの鍵盤(けんばん)の左端(ひだりはし)から順(じゅん)に、アルファベットをあてはめて…」; “Starting from the leftmost key on the piano keyboard, I assigned each key a letter...”; Literal: “Starting in order from the left edge of the piano keyboard, I applied the alphabet...”; 「メッセージの文字(もじ)に相当(そうとう)する音(おと)を、譜面(ふめん)に書(か)きしるしただけだよ…」; “I simply wrote down the notes corresponding to the letters of the message on the sheet music...”; Literal: “I only wrote down on the score the sounds that correspond to the letters of the message...”

Key Points

1相当(そうとう)する = to correspond to

2に相当(そうとう)する links one item to its counterpart

「メッセージの文字(もじ)に相当(そうとう)する音(おと)」 means “the notes corresponding to the letters of the message”.
The に marks what those notes correspond to.

3だけだよ downplays the cleverness

Conan frames the method as simple with 〜だけだよ.
It gives the sense of “that’s all I did”, even though the decoding was actually quite smart.