Why do the Kohanim sing in the middle of Birchas Kohanim?

While saying certain tefillos or brochos, a person may not take a break and remain quiet for the amount of time it takes to recite that particular passage from beginning to end. These tefillos include any brocha, birchos Kriyas Shema, Shema, Shemoneh Esrei, Hallel or Megilas Esther. A pause that constitutes an interruption is calculated according to the time it takes the reader to recite a given section. Pausing from the reading in order to sing is not considered an interruption. It has become customary for Kohanim to sing between each of the brochos of Birchos Kohanim, thus allowing an extended period for the tzibur to recite their supplications without causing an interruption.

ס"ק ג, ד ו־ז, וביה"ל ד"ה קראה; ביאורים ומוספים דרשו, 8 ,9 ו־13

If one took a break in the middle of Shema, does he have to repeat it?

B’dieved, if one took a break in the middle of saying Shema for the amount of time it takes to say all three parshios, he is yotzei according the Shulchon Aruch and he does not have to say Shema again. Rema holds that if he was forced to take the extended break due to halachic considerations (e.g. if he needed to use the restroom or if the area became unclean), he must begin again. If a person took a halachically mandated break during one of the Birchos Kriyas Shema (for the amount of time it would take to say all of the brochos and Shema), he must repeat the brocha that he was in the middle of saying. If he interrupted in the middle of V’yatziv or V’emunah, he must go back to the beginning of Shema.

סעיף א, ס"ק ב, ה ו־ו, וביה"ל ד"ה קראה וד"ה חוזר; ביאורים ומוספים דרשו, 12

If a person interrupted his Shemoneh Esreh due to a hold-up, must he start over again?

If a person took an extended break in the middle of Shemoneh Esrei, the Shulchon Aruch holds that he must start over again, even if the interruption was not halachically mandated. Rema maintains that Shemoneh Esrei would only have to be repeated if one was forced to interrupt it. Unlike the case of Shema, any forced interruption of Shemoneh Esrei--even for non-halachic reasons (e.g. one was stopped by an armed robber)--would necessitate starting again.  In the case of a non-halachic interruption, Rema holds that if a person did continue from where he left off, he does not have to say Shemoneh Esrei again.

ס"ק ד וד"ה קראה; ביאורים ומוספים דרשו, 4

• The words of each parsha of Kriyas Shema must be said in the order that they are written in the Torah. The pesukim must also be read in order.

• A person who either skipped or mispronounced a word must go back to place where the error was made, correct it, and continue with the rest of the parsha. If the word that he omitted was in the middle of a concept, some say that he must go back to the beginning of the pasuk. 

• The pasuk of U’chsovtam is found in both the first and second parshios of Kriyas Shema. If a person was saying this pasuk and forgot which parsha he was up to, he must go back to where it is written the first time. If, however, he continued with lema’an yirbu, it can be assumed that he is in the second parsha.

• Should one interrupt Pesukei D’zimrah to say Shema?

• Must one respond to Shema of the Kedusha of Musaf?

• Does Aleinu have to be said with the tzibur?