There are a lot of compound nouns using an adverb and a past or present participle, Ali. EXAMPLE: A publicly financed service. (adverb + past participle) A commonly held belief. (adverb + past participle). A well-liked man. (adverb + past participle) A rapidly increasing heart rate. (adverb + present participle) An increasingly irritating rash. (adverb + present participle) You might find it useful to look into the different types of compound adjectives we have in English. (A lot of them are similar to ones you have in German).

In the US, the usual term is “soft-spoken.” He’s a soft-spoken man Being soft-spoken is a disadvantage in some jobs. “Softly spoken” seems wrong, or at least inferior, to me. (Possibly this is regional) Of your examples, only “soft voice” seems natural. He’s nearly deaf and she has a soft voice. They often fail to communicate.

Hi Ali In this case "A softly spoken man" is a man who always speaks very quietly, in a calm and even tone.

This is most likely a typo or mistake. The correct word is "soft-spoken." Example: "He is a soft-spoken man. I can barely hear him when he talks." Hope this helps. Good luck!

Interesting, Victor! The British "softly-spoken" refers also to (being) "courteous in speech" (Lexico Dictionary). Thank you all for your helpful comments.