The Weekly Shtikle - Naso
This week's parsha contains the pesukim that the Kohanim recite when they perform Birkas Kohanim. The Shulchan Aruch (OC 128:33), based on the gemara (Megillah 24b) states that someone who cannot properly pronounce the letters, such as one who mispronounces an aleph as an ayin or an ayin as an aleph, should not go up to perform Birkas Kohanim. The difficulty with this is that the pesukim do contain an aleph but do not contain an ayin. Why then would someone who mispronounces an ayin as an aleph be forbidden from performing Birkas Kohanim?
Rashi in the gemara seems to be sensitive to this issue. He gives a specific example of a grievous mispronunciation that would result with the exchange of an ayin for an aleph. However, when explaining the opposite substitution, he writes simply that as a result of this substitution he will disqualify his prayers. This statement of Rashi is quite vague and requires further interpretation but it shows, nevertheless, that Rashi addressed the lack of an ayin in Birkas Kohanim.
The issue is dealt with further in the commentaries on the Shulchan Aruch. Ba'eir Heiteiv raises the question and does not provide an answer. Machatzis HaShekel seems to suggest that this is not an issue as the gemara is simply referring to one who confuses the two letters. Thus, as long as one of the substitutions is significant, it is a sufficient problem.
I suggest a possible explanation for the gemara which may be the meaning of Rashi as well. After the Kohanim complete the main part of Birkas Kohanim, they recite an additional prayer. In this prayer, the Kohanim declare that they have done their part in bestowing the blessings upon the congregation. They beseech HaShem to fulfill His promise to carry out the blessings. In the prayer, there are a number of occurrence of the letter ayin. The most significant is the word "nishbata." If pronounced properly with an ayin, it means "You have sworn." However, if the ayin is not properly verbalized, the word means "You have taken captive." Although this is not part of the actual blessings of the Kohanim, perhaps it is a serious enough mispronunciation to forbid a Kohein from performing Birkas Kohanim.