After plug-in digi+ on my raspi 2, I started volumio and I went in playback setting.
I checked i2s option, but digi+ is not in the drop-down list.
I can find both Iqaudio digiamp+ and dac+.
Do you know what is the best choice for my hardware configuration?
Thanks for your answers.
Math
You could try setting up the DAC yourself following the online manual http://www.iqaudio.com/downloads/IQaudIO.pdf. See especially section 9, and specifically where it says to select the driver in /boot/config.txt.
dtoverlay=iqaudio-digi-wm8804-audio
Presumably this driver has to be provided in the Volumio image, or installed yourself.