Actual answer
1. Can't listen to music over Bluetooth, only through aux cable.
2. Music volume is controlled by your music playing device, not by the A20 itself.
3. IMHO the Bluetooth isn't worth the extra money. It can be quite useful to talk on your mobile (to Ops/despatcher/duty manager) via your headset which keeps you in the loop about what's going on in the aircraft, but its only a very, very minor benefit on an occasional basis.