1, Bluetooth speaker with VOL + - button, is to adjust the volume, usually to increase the volume, just hold down the VOL + button does not let go (let go of the next song), the volume will increase to the volume maximum, usually Bluetooth stereo will emerge a sentence, or drop a few sounds. 2, there is also a Bluetooth audio connected to the audio source device, such as connected to the phone, you can adjust the volume size of the phone, change the Bluetooth speaker output sound size.