I’d like to have it via entities as well.
But I can answer why (considering that the Devs seem to just lurk instead of answering questions): after giving it a shot to code in speed limits via zones enforced by the avatar mixer, and failing to do some within budget with the exception of getting everything to work on client side, I still got quite familiar with the way how things work behind the curtain.
It is due architectural issue inherited from having the audio mixer / assignment client architecture mostly completed prior to zone entities. It requires a refactor or an ugly hack to code to have them cross communicate info on zone entities and changes to them via the domain-server. Zone entities originally were only taking account client sided things such as skyboxes and global lighting, funnily enough. And yes, that means all settings in Zone entities, are still things only enforced by the client. Nothing is stopping modified clients from simply ignoring them.
Only thing that the the assignment clients instances share with each other are the locations of the client being served through the domain-server.
Since Audio Mixer only knows the position of the Audio Listener of the served client, which is uses to do some math to combine all the sounds via the attenuation into a single stream per client, depending on the audio position and loudness. It also know the settings defined the domain-server, hence the “Audio Zones”.
I do know, however that they have something of the sort to get the entity info to the audio mixer in the roadmap, according to @Atlante45 through the worklist.