How to make an UTAU sound softer?

[RaccoonButler]

Not quieter, but like they are singing more softly, given that the voice does not have a soft/whisper vb

 

[sailor _ravioli]

im no expert but you could try to use a vst called charsiesis, i havent tried to use it for that purpose yet, but i think its possible to get a 'whispery' type effect by experimenting and playing around with it until you get what you want

 

[Nohkara]

To make "normal"/power VB sound softer you can...

-use flags! (using BRE50〜90 for example makes it sounds more softy - and breathy)

-try different resampler if you use PC UTAU

-EQ'ing at mixing stage can make a vocal sound softer!
Here is a good tutorial explaining about EQ => http://utaforum.net/resources/mixing-tutorial-equalization.48/

-IF using VB is "Kire style" or in other words "Powerscale" Multipitch VB, try use more its LOW pitches and avoid using it's "shouty" pitches (edit prefix map and/or surffix).

-If possible, try to pick out... Not too shouty/powerful VB. You cannot make very very screaming and shouty VBs to sound very soft and good at the same time.

 

[GothAmaterasu]

I usually use the flags CLE0B70 for a clear/soft-ish sound c:
Flags work differently on different voicebanks though. As Pupuomena said, you may perhaps wish to soften the vocal while mixing. :3