The Mishna in Masekhet Rosh Hashanah (26b) writes that the shofar sounded on Rosh Hashanah has “its mouth plated with gold.” Rashi explains that this refers to the shofar sounded in the Beit Ha-mikdash on Rosh Hashanah, which was especially adorned with a gold plating. The Gemara (27a) clarifies that plating is forbidden in the area where the mouth is placed on the shofar to blow. As Rashi writes, plating in this area would constitute an obstruction, such that one effectively blows the metal plating, and not the shofar itself. Plating is allowed on other parts of the shofar, but not where the mouth is placed. Later (27b), the Gemara adds that plating anywhere on the shofar disqualifies it if the plating alters the sound produced by the shofar.
Rav Moshe Ibn Chabiv, in his Yom Terua commentary to Masekhet Rosh Hashanah, raises the question of why the Sages did not forbid gold on the shofar due to the rule of “ein kateigor na’asa saneigor” – “a prosecutor cannot become a defender.” The Gemara earlier invokes this principle to explain the Mishna’s ruling that the horn of a cow is not valid for the mitzva, as it brings to mind the sin of the golden calf. A cow’s horn is a “prosecutor” – evoking the memory of our nation’s gravest blunder – and thus cannot be used as we seek to arouse God’s compassion on our day of judgment. The Gemara notes that this is the same reason why the kohen gadol wore plain, white garments, without any gold, while entering the inner sanctum of the Beit Ha-mikdash on Yom Kippur, as gold would inauspiciously evoke the memory of the golden calf. Seemingly, then, it would be improper to plate the shofar with gold, as well, for this same reason.
Rav Moshe Ibn Chabiv answers that since gold plating is allowed only if it does not impact the sound of the shofar blast, it is not subject to the rule of “ein kateigor na’asa saneigor.” The gold is an external decoration which has no bearing at all on the sound of the shofar, and therefore, since the mitzva is fulfilled through the production of the sound, and not through the shofar itself, the gold is halakhically inconsequential. It is not the “defender,” insofar as it is not integral to the shofar. This answer was offered by later writers, as well, including the Tiferet Yisrael commentary to the Mishna and the Sefat Emet.
Rav Moshe Ibn Chabiv also suggests a second, particularly bold, answer, postulating that indeed, the Mishna’s ruling permitting gold on the shofar follows the minority view of Rabbi Yossi, who allows using a cow’s horn for the shofar blowing (Mishna, 26a). Rabbi Yossi clearly did not subscribe to the rule of “ein kateigor na’asa saneigor” (or at least not to its application to the mitzva of shofar), and thus in his view, there is no reason to disallow the use of a shofar with gold plating. It is only according to Rabbi Yossi’s view, Rav Moshe Ibn Chabiv suggests, that the Mishna allows fulfilling the mitzva with a shofar plated with gold.
On this basis, Rav Moshe Ibn Chabiv offers an explanation for why the Rambam makes no mention of the Mishna’s comment that the shofar sounded in the Beit Ha-mikdash was plated with gold. Since gold plating is allowed only according to the minority view of Rabbi Yossi, which is not accepted as halakha, the Rambam disregarded this ruling.
Chida, in Machazik Berakha (O.C. 585), questions this theory, noting that it does not seem reasonable that different views existed regarding the practice observed in the Beit Ha-mikdash. The fact that the shofar in the Temple was plated with gold, the Chida argues, is a piece of historical information which cannot be subject to dispute.
As for why the Rambam made no mention of the plating on the shofar in the Mikdash, a number of Acharonim, including Sefat Emet (Megilla 27a), suggest, very simply, that this was omitted because it was a non-binding custom. There was no halakhic requirement to plate the shofar with gold; this was simply a custom that emerged as an expression of special respect for the mitzva of shofar and for the Mikdash. Therefore, the Rambam did not find it necessary to include this practice in his discussion of the halakhot of shofar blowing, where he presents the actual requirements of this mitzva.