I felt like this took a long time to get to the point, which was: Italian Americans are generally descended from speakers of Southern Italian languages that differ from modern standard Italian, and those languages are notable for having terminal vowel deletion, converting "o" into "oo," and voicing unvoiced consonants, leading to them sounding distinct from modern standard Italian speakers.
This gets linked every once in a while, and is worth it every time: http://www.atlasobscura.com/articles/how-capicola-became-gab...