You will need to disable absolute volume in the phone.
First, enable developer options. To do this, go to Device Settings, then scroll down and select "About Phone. " From there, tap on "Software Information" and locate the "Build Number" entry.
Once you've found it, tap on "Build Number" seven times in rapid succession, developer options will then be available on the settings menu of the phone. Go into developer options and toggle the button for "disable absolute volume."
Restart the phone and this should allow you to control the volume of the speakers from the phone.