The only way to turn down the volume for the Bluetooth to my knowledge (on my phone in general) is when you're on the phone via Bluetooth just press down on the volume rocker on the device itself.
2012 Red Candy Metallic/5-Door Hatchback SEL/NAV/Winter Pkg/Rapid Spec/Remote Start
Order Date: April, 28, 2011
Born Date: May 26, 2011
Pick-Up Date: June 20, 2011