"Yo no soy una niña."
Translation:I am not a girl.
I think there is a great misunderstanding here between English and Spanish speakers. I think you mean that you hear the "yo" as in "job" and not as in "yob". These are the sounds /dʒ/ and /j/ in the International Phonetic Alphabet. What sound corresponds to the Spanish "y"? It can be both. We don't differentiate these two sounds. Both are for us a "y" as in "yo", "ya", "yema", etc. It depends on the speaker, his/her accent, the position of the word in the phase, the today's weather :-), whatever. It doesn't matter. One can articulate this letter as one wants, because the listener will always understand "y".
Some Spanish-speaking people in this discussion has understood that you refer to the /x/ sound, as the letter j is allways read in this way. I refer to the guttural sound as in "caja", "jirafa", "ojo", etc. That one that is also heard in German as "ch": Loch, Bauch, Tochter.
By the way, Argentinians usually pronounce "yo" as /ʒo/ as in pleasure, not as /dʒo/, as in job.
I've heard that it could also depend on how fast you speak. I mean, when you're speaking in a normal pace, the pronunciation could sound like "yo" (as in year). but when you talk more slowly, or you want to put more stress on the word, it's more likely to sound as "jo" (as in jar). is that true?