On Shabbos or Yom Tov, if someone forgets to say Nishmas and has already started Yishtabach, should he go back and say it? Does it matter if he already said HaShem's name or started the conclusion? What if he doesn't remember until after davening?
ועוד תקנו לומר נשמת ובמקהלות. ונראה דבכל זה אם לא אמר אין מחזירין אותו ויש לאמרו בקול נעים.
They also decreed that נשמת and ובמקהלות should be said. It appears that if they were skipped we do no make him go back. One should say these things in a nice voice.
The Mishna Berurah at the end of siman 282 brings the Darkei Moshe that one does not go back if he skipped Nishmas, but is allowed to say it after Shmonah Esrei if he wants. In the Biur Halacha he says if one remembers that he skipped it before he says Baruch atah Hashem Kel Melech etc, he should go back and say it in order.