Your problem is that your characters aren't rounded. They don't have distinguishable voices because they aren't distinguishable people.
Do this as an exercise: Pick your favorite TV show, movie, book. Pick two or three characters from each. Interview them. For example:
What's your favorite book?
John: Hitchhiker's Guide to the Galaxy. Read it to tatters in Afghanistan.
Sherlock: "Favourite" would imply affection. I do not feel affection for inanimate objects. If you are asking which book I find most useful, that varies from case to case. A group of Chinese smugglers once used London A to Z as a cipher key —
John: Yes, let's not revisit that case, shall we? I ended up tied to a chair with a crossbow pointed at me because they thought I was you.
Wilson: I'm really enjoying Game of Thrones.
House: So am I. By which I mean the show, with all the boobs, not the big doorstopper novels.
Wilson: Boobs and politics.
House: Politics and boobs. Best of both worlds.
Go through your "interview questions" or find the email surveys which go around and practice interviewing someone else's characters. It's a bit fanficcy, but it will teach you how to look at a character and answer in someone else's voice.
Then you need to figure out the why. Why does Sherlock not have a favorite book he returns to? What does John get out of rereading something he knows by heart? Does House find the act of reading boring or too time-consuming? Does Wilson enjoy the soap opera or the intricate politics?
Once you've taught yourself to analyze an existing distinct character, you can use those tools to make your characters more rounded. When you have a character who feels like a real person, it's much easier to have that person speak in his or her own voice, which is not yours. Remember that your motivation and your characters' motivation are not the same.