Something to remember, too, is that readers aren't going to hear the voices the way you do. When I'm reading, I don't even consider what the characters actual speaking voices are like at all. In my mind they all have my accent and, I suppose, my mental voice. The more I try and explain that the harder it is, haha, so, I suppose, read a good chunk of dialogue and then think about if you 'heard' it or not. Probably not really. I'm not even sure what my character's voices sound like, even when I can see and hear the scenes perfectly in my head. They all have 'mental voice'. They talk differently, using different words and syntax etc., but I wouldn't recognize their voices alone. I know their accents, which aren't mine, and since they're English and I'm Canadian they say stuff like "I'll not" rather than "I won't", etc., but I don't even think about their accents when I read. And that's my own characters!
I'd say, if the way a person talks matters, mention it once, in the context of why it matters, but remember characters will remember the implications of the way they talk rather than hear it. In The Sun Also Rises, by Hemingway, a couple characters are American, a couple are English, and one is Scottish. A few words and a bit of grammar differ between them, but just barely and only if you're paying attention. I could tell you who belongs to which country, but when Brett (English) and Jake (American) are talking, I don't even think about it.
It's possible all this is just me, and everyone else hears way more variety when they read (if so, I'm jealous), but I think most of us have pretty generic voices in our heads. So don't worry too much over making sure we hear the one you want us to, because we won't, haha.
(I hope this makes some sense-- I feel kind of discombombulated after a long day at the, er, retail store (can't wait till grad school!). If not, just ignore me, haha).