As someone who dearly misses her ability to actually talk aloud, and of an even split Irish and Scottish heritage... The scot in me says you're kind of right, and the Irish part of me, where my accent properly lay, says that the thing to remember is that English is not our native language, not truly. Our own natural languages got driven out by english control - with kids punished in school for speaking them, and worse; there were even laws passed banning their use with the active intent to 'extinguish their sinister laws and customs'. They didn't fade away naturally, they were actively killed, or a good attempt made thereof...
Er, ahem, sorry. At any rate...
Different tones and timbres are the things that should differentiate voice sets, as others are saying. That was kind of what I meant (though I probably chose the wrong word) when I mentioned archetypes before; the bright and bubbly, the rough and gravelly, etc.
