The Gina Carano vocal mystery is OFFICIALLY OVER -- because the rib-cracking MMA fighter has finally admitted that her voice WAS altered for her role in the movie "Haywire."

Carano appeared on a podcast for the "No Holds Barred" MMA blog yesterday ... when she explained why director Steven Soderbergh felt he needed to do some "altering and mixing" to her voice.

Long story short -- Carano says Soderbergh wanted people to believe that Gina and her character are "two completely different entities" ... and felt the vocal change would help.

Carano added, "I'm just honored to be a part of it ... there was a little joke that I was saying that even if [Soderbergh] would have shaved my head and put Bill Clinton as my voice, I would have still done it."

Gina Carano is the REAL DEAL in the world of MMA ... but there are allegations she's not totally the real deal on the big screen ... because her voice in the new movie, "Haywire" ... may NOT be her voice at all.

Sources connected with the production tell TMZ ... the folks behind the movie didn't like the way Carano sounded when they filmed the flick ... so they changed it in post.  It's unclear if they used fancy electronics to alter Gina's vocal quality, or whether they dubbed in another female voice.

Monday night, Carano was at The Grove in L.A. ... where she totally spoke with us ... and sounded great ... at least that's our opinion.

We placed multiple calls to the production company ... so far, no comment.