Slave Volume coding

Hi Guys

I want to ask a question about Bluesound. I have created a Blusound Driver. Everything went well, but finally I found a problem when the Bluesound is grouped, the Slave volume can't be controlled individually. In Bluesound status, Slave volume goes with Master.

but I finally figured out how to find out the slave volume by:

and get a reply

volume db="-58.6" mute="0" offsetDb="0" etag="8a1108f6e68a27f2af3f4f5878aa6e50">5

My question, how to code it so that I can control the slave with the data I receive.

Thank you.