This is often the case with many makes/models of TV when controlling the audio via the HDMI-CEC protocol.. I have an LG TV which just shows a ‘+’  or ‘-‘ when changing volume.

Same for me…. so do u think there is no way to fix it?

Thanks anyway for your answer!

I’m not aware of any, but if you have an LG TV there is an ‘engineers menu’ that allows some settings to be changed. I haven’t looked into the matter, as altering such settings can invalidate the manufacturers warranty. I’ve just left things as they stand and just set the volume level by ‘ear’.

I just purchased an LG 43UP81006LR and am equally annoyed / disappointed that it will not display volume level of the Beam .  

Especially when the Beam worked perfectly with my 7yr old  Samsung UE40H6400. 
So it’s not an issue with the Beam - the ARC HDMI-CEC is fine. 

Would exchange the LG for the similar Samsung UE43AU9000, but from this and other Reddit forums, there’s not much assurance the new Samsung new model will have this function. 
I would love to know if it can be enabled in the software of either TV!
 

Sonos doesn’t write the software for the TV sets, you may be better off asking in an LG or Samsung forum. 

Neither of my Samsungs (from 2011 and 2019) has displayed numbers when changing volume on my Beam.