Meaning: This sentence describes someone producing musical sounds with the voice inside or within a place where you wash yourself with water.
Choice of Words: The word "sang" is the past tense of the verb "sing," which means to produce musical sounds with the voice. "A" is an indefinite article that means one. "Song" is a piece of music with words that is sung. "In" means inside or within. "Shower" is a place where you wash yourself with water.