So, new clip featuring Makini: https://www.youtube.com/watch?v=ObMIMyfU-Dw
She's kind of annoying (I personally don't really like these kinds of characters anyway, particularly in 'kid' shows) but I guess it confirms in-universe that Mufasa and the other "great kings" can make themselves visible and audible to whomever they want at any given point, so whenever Kion or Simba or someone else - without the training to hear them, I suppose, as in the case of those in Rafiki's position - sees and talks to them, anybody not keyed in on the conversation just hears that person talking aloud to empty space. (Would explain Nala's seeming dismissal of Simba's "seeking council from the Great Kings" with her "Did they help?" response in their little scene in Simba's Pride; she hasn't had that experience so she wouldn't know whether it's true of not.) Not sure if Scar works the same way; we'll have to see.