The Mishnah (Berakhos 15a) records a dispute over whether you can fulfill your obligation to recite the Shema if you are not careful in how you pronounce it (kara ve-lo dikdek be-osi’oseha). The Shulchan Arukh (Orach Chaim 62:1) rules that you fulfill your obligation after the fact (bedi’avad). While there is room to discuss the exact definition and parameters of the care required in pronouncing the words, it is certainly appropriate to at least try to correctly pronounce every letter and vowel.
When I learned how to pray, the siddur I used marked the sheva vowel so that users can distinguish between the sheva na and the sheva nach, and pronounce them differently as is proper. However, when using the recently published Koren Sacks Siddur, I noticed that a vowel that in my previous siddur was marked as a sheva na, in this siddur was marked as a sheva nach. Since this is in the Shema, it is of even greater importance than elsewhere. Plus, the word appears twice in the Shema. Every morning and night I recite “u-ve-shokhbekha” in the Shema when, according to the Koren Sacks Siddur I should be saying “uv-shokhbekha“. I initially thought that this was an error in the new siddur but I checked in the Tanakh Simanim, which differentiates between the sheva types, and it matched the Koren Sacks Siddur.
I contacted Koren to inquire about this and they put me in touch with their grammarian. Here is my summary of what he told me (please keep in mind that I might be slightly inaccurate due to my own ignorance). When a sheva follows a long vowel, it is a sheva na. While a shuruk (“u“) is usually a long vowel, when it appears at the beginning of a word it is not. However, there is a lesser known vowel sign called a meseg or ga’ayah, which appears under the shuruk of “ובשכבך”. This sign indicates that the shuruk is not a short vowel but a long vowel. Therefore, the sheva following it should be a sheva na and the word should be pronounced “u-ve-shokhbekha“. Up to this point, I had already understood. That is why I asked Koren.
Their grammarian explained to me that there is an unusual construct in a word in which the ga’ayah is called a ga’ayah keveidah, as opposed to the ga’ayah kallah described above. In such a word, the ga’ayah does not indicate a long vowel and the vowel remains, as expected, short. (The requirements are so complicated, they make my head spin. If you want more details see R. Mordechai Breuer, Ta’amei Ha-Mikra, pp. 202-203; Prof. Yisrael Yeivin, Ha-Mesorah La-Mikra, p. 212.) “ובשכבך” is such a word. Even though there is a ga’ayah under the shuruk, the following sheva remains nach and the word must be pronounced “uv-shokhbekha“.
I asked my friendly neighborhood linguist about this and he was shocked that I even considered it a question. When I asked him whether I should change how I pronounce the word in Shema to match the Koren Sacks Siddur, he said that I absolutely should do so immediately.
UPDATE: Commenters have pointed out that there is a debate over this and the Siddur Eizor Eliyahu says that the Vilna Gaon pronounced it as a sheva na. My friendly neighborhood grammarian, however, said that he doesn’t trust that report in the name of the Vilna Gaon. I also want to bring the following text to your attention, in which R. Eliyahu Levitas says that the ga’ayah (which he calls ma’arikh) should not change the pronunciation in this word. It comes from from his essay on the ma’arikh at the end of his comments to Minchas Shai.