It depends on the use of the word "soft". In the sentence "the pillow was so soft" it has a different meaning to "quiet"
With the sentence "Her voice was so soft" then "soft" can be a synonym of "quiet"
But, someone who is softly spoken is not always quietly spoken. A soft voice can also mean "soothing" or "calm". Even when "soft" is used to describe sound, it does not always mean exactly the same as "quiet"
Quiet.
A synonym for "whispered" is "murmured".
noiseless,inaudible,low,soft,discreet,soundless, and unobtrusive
Secrecy, confidentiality, solitude.
noiseless, quiet, silent
relaxation,relax,quiet
mushy
Soft
smooth
Soft
Quiet
quiet, satisfy, mollify, pacify