The Trumpets And The Lyre (Behaalosecho 5767)

And God spoke to Moshe saying. Make for yourself two trumpets of silver - make them beaten. They shall be for you to call the community and to make the camps journey. And blow on them and all of the community shall assemble to the door of the Tent of Meeting.... And when they come to war in your land for any trouble which afflicts you and you shall trill on the trumpets and you shall be remembered before the Lord your God and you will be saved from your enemies. And on your days of rejoicing and your festivals and your new moons, you shall blow on the trumpets over your burnt offerings and over your peace offerings. They will be a remembrance for you before your God. I am the Lord your God. (BeMidbar 10)

Rashi quotes a Midrashic tradition that the phrase ‘make for yourself’ indicates that the trumpets were only to be used by Moshe:

Make for yourself two trumpets of silver - that they should blow them before you like a king. Make for yourself - from your own make for yourself - you shall make for yourself and for no other; you shall use them and no other shall use them. Know that Yehoshua the pupil of Moshe did not use them, rather shof’rot .... from which we learn that Yehoshua his pupil did not use them. And not just Yehoshua, rather even Moshe Rabbenu, while he himself was yet alive, they were concealed.... (BeMidbar Rabbah 15:15)

A similar Midrash indicates that the reason only Moshe could use them is that they were the appurtenances of the Jewish king, a role that only Moshe fulfilled:

Make for yourself - you shall use them, for you are the king, but no other shall use them, except for King Dovid, as the verse says, ‘And the Levi’im stood on their platform, the singer sung and the trumpet players trumpeted.’ Rav said, ‘The trumpets in the Temple - they too were concealed, but King Dovid used a lyre, as the verse says, ‘Awake my honour, awake the harp and lyre, I will awaken the dawn.’ (BeMidbar Rabbah 15:16)

We see from this that there is even a disagreement as to whether the trumpets could even be used by King David. It seems that Moshe may have been in a category of his own – simultaneously the war leader, King and spiritual guide of the Jewish people, a role that even King David may not have fulfilled. The Sochatchover offers an insight:

The issue is that behold shof’rot indicate humility as in the Yerushalmi - Considers us as though we bellow like animals before You.... Trumpets indicate status. Therefore Moshe who was the most humble of all men, did not need to stimulate humility through the shofar and he alone could use the trumpets .... But in war, they needed the trumpets for opposing the enemy and opposing outside forces, they needed to strengthen themselves. It would not be right to be humble, for that might result in them nullifying themselves..... And as the Midrash says, ‘My dove - with Me they are like a simple dove - whatever I impose upon them, they do and listen to Me. But with the nations of the world, they are tough like wild animals.’ (Shem MiShmuel 5670)

King David needed to express that humility: indeed this was a constant theme throughout his life. Moshe, on the other hand was already the very embodiment of humility and thus could use the trumpets to express his status as leader without fear of error.