oups another hint may be… it seems there are problems with volume control with the firmware 4.6.415145.35
see Volume control issue.
For compatibility reasons with other devices form other brands I also have based on Linkplay31, I blocked all my units to a much earlier firmware version, so I can not testify… But worth trying on your side what the fellows of the other post did i.e. reset to factory settings via the app. Even if this only temporarily fixes the issue, then you would confirm the PB with the 4.6 415145.35 version…