Is "和声细雨" ever an acceptable variant of "和声细语"?
The copy writer using "和声细雨" instead of  "和声细语" is an unforgivable mistake 

The correct idiom with 细雨 at the end is  微風細雨 

The correct idiom with 和風 at the beginning is '和風麗日' ( variant of  '風和日麗')

Many common idioms can be rephrase for artistic reason, but the characters in them cannot be changed

Example:

Standard: 風和日麗 (wind is gentle, sunshine is pleasant)-- variant: 和風麗日 (gentle wind, pleasant sunshine)

Standard: 窮山惡水 (hostile mountains, vicious water -- variant: 山窮水惡 (mountains is hostile; waters is vicious)

Standard: 鴻圖大志 (grand ambition, high goal)-- variant: 志大圖鴻 (goal is high, ambition is grand)