Adjusting Volume From iPhone/Google Audio Cast Combination

[mdindestin]

Plan

Premium

Country

USA

Device

(iPhone 7 with google audio cast)

Operating System

(latest iOS.)

My Question or Issue

We listen to spotify via speakers connected with 1/8" cable from google audio cast "pucks" to the aux input of the speakers.  It's a great whole house system except one issue: 

When we want to adjust the volume, and select the side volume buttons on the iphone, the volume may just turn down the phone itself which has no effect on the music output.  Other times, it turns down the speakers on the speaker group. 

Is there a consistent way to turn down the speaker group volume while spotify is playing?