Adding and subtracting volumes shouldn't be allowed in the first place; you should only be able to multiply (and maybe divide by abuse of notation) them, precisely because it keeps the meaning intuitive: if you have a 0.5 volume and want to decrease it by 0.5, you want 0.25 at the end instead. It's the same reason why you can't add/subtract percentages either.

In the context of an audio slider, I certainly can subtract percentages: I start at 100% and go down in 10 steps of 10%

Though you will rightfully point out that using linear instead of dB here is misleading anyways, as hearing is logarithmic

Adding and subtracting percentages is not a well-defined operation. If you're at 0 % and subtract another 10 %, should it give you -10 % ? Depending on the meaning of the percentage it might not make sense at all.

My point is that much like adding percentages is almost never the correct thing, so it is with volume. With volume specifically, adding and subtracting only applies to specifically linear values (with a bunch of caveats as we have seen) but in the general case you can only multiply and divide.

You can work with "10 steps of 10 %", and do whatever you want to your f32, but once you wrap it into a Volume::Linear, it then loses the ability to be added to.

The drawbacks of manipulating linear volume values are besides this point (though additional motivation to prevent footguns). This is more fundamental.

And for the implementation of an audio fader specifically, this is something that was discussed recently for the new official 2D template: you can find a code snippet here: TheBevyFlock/bevy_new_2d#385 (comment)

Fair enough, I think you've convinced me :) Renaming the issue.
The template is second party, not official, btw