Garmin's needs to be involved cause that is where the tunes are. I tried pairing the phone with Sena but then no tunes or GPS. Haven't tried pairing Patti's phone to her set. Maybe today it will be in a good mood.
Anticipate disappointment. It's not Garmin's or Sena's fault. The link between the Garmin and any BT headset uses the HFP (handsfree) protocol, which is meant only to pass voice and minimal control data. This is a mono sound link; music will have only one channel, period. The way to audio happiness is an A2DP link (full stereo, controls). Smart phone to BT headset uses this.
BUT... there is zero volume control at the source end. There is only one volume level: 100%. Again, this applies to
any GPS receiver, not just Garmin. (Some people claim they can control BT volume - performance talks, BS walks, ain't seen performance, yet)
My workaround is to use an Android play app that (in effect) goes to 11. Or louder. Android limits volume (to protect little ears from blowing out eardrums), but it can be by-passed. While, in some regards, a PITA to live with (e.g., shuffle options are bizarre), it does do "to 11 and beyond!". Crank up the phone, use the GPS, and turn the headset volume down. Easy-peasy. Sorta.
AFAIK Apple users are SOL on this workaround. Teaches you to not have the need to pay Apple far too much for a white brick, don't it.