Not sure if it runs a random voice (since I couldn't hear anything when I tried) but Person Data does have an extra field "Chat Balloon On?" which is set to a value > 0 when a sim is talking. Only really seems to do anything when the sim is standing idle, but the "group talk" from TS1 where the sims talk to each other while doing other interactions like eating, watching tv looks like it should be looking at this (has been ported to "do online group talk"), but it's looking for a very specific value which I'm not too sure about. I'll look into it.