Fix ESS mixer volume handling

[johnnovak]

Description

Fixes issue reported in #3700 (comment).

86box seems to have a more correct ESS mixer implementation, although it's unclear whether that's accurate to the real hardware or they just guessed the values.

https://github.com/86Box/86Box/blob/e61c6205584e4414fc4a64494be5c6f1aab3e6ff/src/sound/snd_sb.c#L93

In any case, reconciling the ESS and SB-specific mixer volume handling would take some non-trivial effort, and I'm not going to complicate our Sound Blaster code with that for the sake of a single 2024 demo. All in all, it's a rather insignificant detail to get the volume curve exactly right.

Manual testing

Now you can set the audio volume with the + and - keys in koolnESS: https://www.pouet.net/prod.php?which=96920

Checklist

Please tick the items as you have addressed them. Don't remove items; leave the ones that are not applicable unchecked.

I have:

• followed the project's contributing guidelines and code of conduct.
• performed a self-review of my code.
• commented on the particularly hard-to-understand areas of my code.
• split my work into well-defined, bisectable commits, and I named my commits well.
• applied the appropriate labels (bug, enhancement, refactoring, documentation, etc.)
• checked that all my commits can be built.
• confirmed that my code does not cause performance regressions (e.g., by running the Quake benchmark).
• added unit tests where applicable to prove the correctness of my code and to avoid future regressions.
• made corresponding changes to the documentation or the website according to the documentation guidelines.
• locally verified my website or documentation changes.

[weirddan455]

So one small correction in the first commit and the rest are refactors. Looks good.

[johnnovak]

So one small correction in the first commit and the rest are refactors. Looks good.

Yep, that's it! Thanks, man! 🎉