In Windows 8/8.1, when I push the volume up or down controls on my keyboard, it changes volume by 2%. 2% is too loud or too quiet, and I often need it to be a 1% difference but I can’t seem to do that.
Is there a registry key or similar that changes this step interval?
You must have very sensitive ears or very powerful speakers to be able to tell the difference.
If it’s the latter, do you have a hardware volume control? I run a 5.1 speaker system with a desktop audio control, and keep the hardware set at about 40% volume so that the software control represents a reasonable control range. (I turn it up when I am doing audio work.)
As nearly as I can tell, 50 steps is fixed in the OS. You might have to go to a more sophisticated sound card and driver to have finer control.
I have a USB speaker bar and it goes straight from too quiet to too loud at night. It’s just a minor inconvenience, like so many other issues with windows. Thank you though.
It might be a fault of the speaker system itself. The volume control is meant to work directly with an audio subsystem, and going through a USB link might frag that somehow. You might see if there’s an updated driver for the speaker bar, or an optional one that works better than default OS-USB level control.
But of course anything that doesn’t work must be Windows’ fault. 
The per-increment decibel increase is probably decided by the speaker system – hardware and drivers – but the steps between each increment (2%) is apparently hardcoded into the OS. That’s Microsoft I’ll take a look at the speaker drivers to see if there are any updated drivers, but I doubt it since it just uses the standard USB audio libraries. Yeah, nope 
And I blamed it on Microsoft because I never seemed to have this issue with Windows 7 (unless I just forgot), and, 8 has so many other minor annoyances that I’ve just had to learn to suck up – that’s a whole 'nother thread, one that’s been done to death.
It is apparently possible to override the keyboard hotkey for volume control and attach a third-party program to control it, but that just seems overkill for this minor UI issue. If I drag the slider bar with my mouse and set it to 1%, it’s fine. Just tedious.