I suggest it is made more clear through an announcement or so which familiars/animal companions can speak and which can't. I've seen at least two speaking balls of light and a speaking blaze hound in an hour or less, so there seem to be some confusion about this.
Yep. Seconded. Same for polymorphed folks, when (if) we get there...
I believe this information was made available, and I think it even made it into Snoteye's Great FAQ. Obviously that information is not as readily available with the new forums and all, but to the best of my recollection, only things with vocal chords capable of speech (pixies, imps, etc.) can talk. Shining lights can't but I may be wrong.
You're right. I don't think this warrants an actual announcement, though. If you see it happen, point it out to the player and/or direct them to a DM.
Yes... In real life, only things with vocal chords can talk. But this is NwN, there's ghosts, they can talk, there's dragons that breathe fire, and quite a few people on the server can say some things, wave their fingers, and something explodes (aka: Fireball). Not everything has to make sense, though if you REALLY want to get all technical, then the way the shining light could talk would be by vibrating rapidly, causing vibrations in the air, otherwise known as sound.
Though since all the familiars and animal companions are getting changed soon anyway, to more tropical island themed creatures rather than demonspawn from the underdark, I don't think it's that much of an issue right now. The only problem (if it can be called that) is that people have chosen to take a familiar that has 1hp per level, an AC of 15, and concealment and want to RP with it.
Yes, people COULD indeed emote it dumbly floating around (as people have pretty much ordered me to do), like a baby that can't speak yet pointing at the cheerios he wants trying to get people to understand that he wants some cheerios, which could be fun, for a few seconds, but would soon get boring because other people would soon lose interest, and go on to talk with the person with a dragon for a familiar, and we'd be stuck there, with our little ball of light that will certainly die should someone mistake it for a will-o-wisp and decide to kill it, causing us to lose 100xp (I myself have lost some 500xp or so from people killing it, not including the times monsters have killed it). I really don't see why people are upset about this, people take the Ball of Light familiar either because (in my opinion, that is) it's the only good aligned familiar, or because they are taking it for RP reasons, because it surelly won't help in a fight, that nice 1dmg per hit (and a +0 to hit, at that), 15 ac, and 1hp per level will get you nowhere.
Dragons have vocal chords. Bobbing up and down is not conducive to intelligible speech (not even if you only have 3 HP).
Of course, there is the fact that it's a floating ball of positive energy, and that it CAN do things other than just bob around and get mashed by enemies. Investigate the radial menues more.