additionally the dom.audiochannel.mutedByDefault and media.default_volume settings may also be useful if you want slightly different behavior.
If you want a more blunt tool you can also use content blockers to block media content until you opt into it for a particular site.
> I know there are plugins than can somewhat do this, but no solutions are universal or great.
How so? There are about a dozen FF addons covering different use-cases like muting inactive tabs or all tabs but a designated one. If none of them meet your particular expectations that might also indicate that everyone wants something different and it is difficult for browser vendors to cover all expectations. Maybe you should modify an existing addon instead to do what you want.