Adjust volume based on numerical values for consistency

You can use a program like Sonoseqencr to set each zone to specific numerical value (button). Then adjust the main slider in Sonos only to lower or increase volume.

Numerical values are of not much use to achieve consistent sound levels. For one, you need different values across different Sonos models of just play units to get these. And two, sound levels vary across and within sources for a given numerical value of the volume control. You will still end up doing the same back and forth of the slider from time to time to get consistent sound levels.

I agree

Numerical values are of not much use to achieve consistent sound levels. For one, you need different values across different Sonos models of just play units to get these. And two, sound levels vary across and within sources for a given numerical value of the volume control. You will still end up doing the same back and forth of the slider from time to time to get consistent sound levels.

True with respect to source material (although the current loudness wars / "brick-walling" trend helps consistency), but visible volume levels are very useful as an arbitrary reference point. For example, I know that normal daytime listening levels for modern mastering is a "30" on my amp for one of my Sonos zones. I have no idea what that is on Sonos for my other rooms. And therefore no idea what to return to when listening to something mastered differently to a different reference level.

Still, at least unmarked "just use your ears" volume controls look pretty and minimalist. I suspect this is one of the main reasons why most music apps have unmarked volume sliders.

I know that normal daytime listening levels for modern mastering is a "30" on my amp for one of my Sonos zones. I have no idea what that is on Sonos for my other rooms. And therefore no idea what to return to when listening to something mastered differently to a different reference level.

Still, at least unmarked "just use your ears" volume controls look pretty and minimalist. I suspect this is one of the main reasons why most music apps have unmarked volume sliders.
If they were available on Sonos, there would perhaps be half a dozen other numbers you would have to also remember in a multi zone system. While the reason you mention is probably the reason why Sonos does not provide this information, I also suspect that adding it would not provide a lot more value/speed compared to eyeballing the slider, that has a small range of motion to start with, to a familiar position while also adjusting it in sync with what the ears are telling you.

If they were available on Sonos, there would perhaps be half a dozen other numbers you would have to also remember in a multi zone system. While the reason you mention is probably the reason why Sonos does not provide this information, I also suspect that adding it would not provide a lot more value/speed compared to eyeballing the slider, that has a small range of motion to start with, to a familiar position while also adjusting it in sync with what the ears are telling you.

There are two volume levels I need to care about with Sonos: the level on my Connect (through my amp), and the master volume level on my all-in-one systems (they're all OK relative to each other). But yes, where so many permutations of different multi-zone components are concerned, this is a non-trivial UI design problem to solve.