I have read about muting all pico system sound the hard way, but I do not care about all sounds.
My application (developing with Unity for Pico G2 4K) has its own Volume+/- sound-FX, but I keep listening to both Pico's SFX and mine.
The best solution is to disable Volume Sounds, when the application is active, via the Unity SDK.
If the Volume+/- SFX cannot be disabled all alone, then disable all system sounds, while our application is active.
If there is no system control via Unity SDK, maybe there is a way, with Console commands (like in the aforementioned post) or an android option in the android menu -but- has to be only about volume+/-.
Can either of the above be done?
From our understanding, you want to press the volume key not to play our sound, but just to play your sound. However, sorry to say that this sound cannot be disabled. You can only set the volume with our SDK.