It's normal, as much as anything connected with writing can be normal, to hear the characters' voices.
However, I would caution you against picturing yourself as the MC, unless you're writing an autobiography. Strictly speaking, there's nothing wrong with picturing yourself as the MC, but the danger is, if you write your MC as yourself, she's going to end up being an Mary Sue (or Marty Stu, if you're a guy). She really needs her own personality, her own likes and dislikes, her own appearance, her own everything. She can't be you in disguise, because readers can tell if your novel is nothing but a wish fulfillment fantasy.
But yeah, hearing your characters' voices in your head as you write is generally normal. It's only if you hear them when you're not writing that you should start to worry. . . .
The bolded is likely to draw some ire, I think.
It's only really a problem if you lack the mindfulness to keep your own negatives and positives in balance. There's a natural tendency to exaggerate one's positive traits, and try to erase the negative ones, which is why it's so easy for self insertion to slide on down to Sue town.
It's totally possible to avoid that, though.