It's probably better to go back to character and decide who each of your characters are and that should help you give each of them a distinctive voice.
Once you've got a grip on each character, they'll develop a life of their own that will become independent from your own authorial voice. You'll know when you're there when you find that you as the writer want to say one thing and you can feel your character staring back at you from the page, saying 'Excuse me, but I wouldn't say that' or perhaps 'Oi you dozy old mug, I don't yabber on like that, what you take me for? A posh git!' Or something along those lines.