As noted in an earlier edition of Daily Halacha, on Shabbat we receive an extra soul – one on Friday night and another on Shabbat morning – which descends upon us in three stages. On Friday night, the first stage occurs as we recite, "Bo’i Kalla"; the second during "Barechu"; and the third just before the Amida, when we recite, "U’fros Alenu." On Shabbat morning, the three stages occur at "Nishmat," when we begin the Amida of Shaharit, and when we recite "Ayeh" in the Kedusha of Musaf.
Interestingly, at each of these points of the prayer service we stand in order to receive that component of the "Neshama Yetera" (extra soul) – with the exception of "Nishmat." For some reason, it is customary to sit during "Nishmat," as opposed to the other five points during the prayer service, when we specifically stand in honor of the "Neshama Yetera," and the question arises as to why we do not stand for the recitation of "Nishmat Kol Hai."
In truth, the Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1867-1939) writes (56:23; listen to audio recording for precise citation) that one should, in fact, stand when reciting "Nishmat," for this very reason. Although this is not the accepted custom, it does have an explicit source, and in some Siddurim of the Ben Ish Hai it is written that one should at least briefly rise, if only slightly, when reciting the words "Nishmat Kol Hai," to give honor to the "Neshama Yetera." We certainly do not seek to oppose the common custom to sit during "Nishmat," but at the same time, we should be aware that according to some Halachic authorities, it is proper to at least make some gesture when reciting the words "Nishmat Kol Hai."
Summary: It is customary to sit during the recitation of "Nishmat" on Shabbat morning, but according to some opinions, it is proper to stand, or to at least rise slightly from one’s seat when reciting the words "Nishmat Kol Hai."