Most of the posh sounding characters are posh, or at least formally educated. Gale is a wizard (clear enunciation for spellcasting), Astarion ws a magistrate, Wyll is the son of a duke, Minthara is a princess.

Karlac and Halsin aren't posh and don't sound posh. I'm not sure about Lae'zel, but that just leaves Shadowheart. Maybe Shar has a thing for accents?