How is your Beam connected to the TV set? What is feeding the signal to the TV set? What does the Sonos controller say that the Beam is playing?

And clear up what you mean by “locking level control between”, please. The TV should only be playing through the Sonos Beam, and not at all through the TV set’s speakers. 

I don’t recall exactly were it appears, but there is an option in the TV to disable it’s Volume control. I think that you are currently using both the TV remote and a SONOS controller or another remote (cable box?) to control BEAM’s Volume and these controls fight each other. You can get into a situation where the TV or BEAM Volume is maxed, yet there is not much output or there is not much control range. You could get into a situation where the Volume is much too loud or much too quiet and there is apparently no intermediate setting.

Are you seeing any numbers displayed on the TV while you are adjusting Volume? If so, look carefully, because there may be a different set of numbers depending on which controller is being used. This indicates that Volume is being controlled by multiple devices -- it’s a “war” and that the operator usually loses.

I fitted a blank headphone jack plug in our Toshiba, automatically cuts all TV speaker audio.

On HDMI-ARC the speakers of your TV should quit the moment the Beam is connected.