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
noiseless, quiet, silent
quiet, breathed
The synonym of privacy is quiet.
noiseless,inaudible,low,soft,discreet,soundless, and unobtrusive
relaxation,relax,quiet
Soft
smooth
Soft
mushy
quiet, satisfy, mollify, pacify
Quiet, hushed, noiseless.