If you want to adjust the volume of your piano, check out the Mixer option from the Pause menu. This feature is available as long as you connect your piano to the device you use.
To access the Mixer, tap or click anywhere on the song screen and select Mixer from the Pause menu.
Please note: wireless technology can be too slow when used with musical instruments. This can lead to increased latency. The Mixer feature stops working when the app detects increased latency.