How to turn off boot up chime on late 2013 mac pro?
The boot chime is always as loud as the volume when you turn off your mac. There are tools for mac to disable it I believe. I don't know wether those affect windows as well. 

EDIT: It seems the volume you set in windows has no control over the volume when booting again. So if you only use windows anyhow, boot into Mac OS (by holding the alt-key when booting). Then set the volume to zero (turn it down, don't use the mute key). You're Mac shouldn't chime anymore.