I previously had a Sonos beam and the arc produces less sound volume at same level than the beam. 

So what exactly is the problem you experience? Just that it needs to say 50 where the Beam needed 20?

I find the YouTube app on my TV needs the volume about 20% higher than set top TV boxes.  I don’t notice a difference between my Beam and Arc.

The problem is when I switch between streaming apps I will blow myself with the sound way to loud because of the disparity in required volume level. How can I normalize the actual decibel of sound across the different apps? I also don’t think the arc produces the same decibel of sound as the arc on the same volume setting. 

On the differing sound levels I think you need to take that up with the providers or their regulatory bodies; it frustrates many of us. 

Different speakers, placements and orientations may affect the perceived volume from a specific listening position. Sonos don’t provide any “power rating” for the amps in their speakers, so the volume number 20 on a Beam may well need 30 on an Arc to produce the same volume. There may be some reviews that give those figures.