Hello Sol, You can use smooth to describe the sound of a voice, but not in the way you have written ("Smooth tone of voice.") First of all, a person's tone of voice has to do with how he or she says something, and this can change. When referring specifically to the tone of voice, I find it is more natural to use adjectives like friendly, nervous, angry, which could also describe a person, or those specific to speech like conversational, interrogative, demanding. Other adjectives are used in more technical uses of English, such as in communications. To speak about the sound of someone's voice in general, we usually use adjectives like smooth, deep, rough, monotonous etc. Some natural ways to write this are: "He has such a smooth voice" "Her voice is so smooth that I could listen to her all day" "His voice sounds so smooth on the phone" Hope this helps!

Yes! Soft-spoken is also a good choice :)

Thanks Kelsey :)

Yes you can . Normally when you describe someone's voice as being smooth it means his voice has a soft and rich texture .