For me, well, I thought of this question because it was only recently that I realised that the Spanish voice in my head is male (unlike me) and actually sounds completely different from my voice. It makes sense though; it's my husband's voice, he is a native Spanish speaker and I learnt much of what I know through interacting with him.
My other languages are more complex. Japanese switches between my own voice (with possibly a slightly better accent than I actually speak) and a few different types of native voices. What I'm wondering about is why I sometimes hear such vastly different voices when I read. I don't speak with many native speakers of Japanese these days so why would the inner voice change completely from one week to the next? Are they just different remembered voices?
For Korean I think the voice I hear is an actress from a tv series I watched quite a lot of. In French it's just my own (rather bad) accent. The other languages seem to be mostly a mix of the recordings I have been using to study them.
What about you guys? :)