Adam Clarke on Joshua 6:4

Seven trumpets of rams' horns - The Hebrew word יובלים yobelim does not signify rams' horns; (see the note on Lev 25:11); nor do any of the ancient versions, the Chaldee excepted, give it this meaning. The instruments used on this occasion were evidently of the same kind with those used on the jubilee, and were probably made of horn or of silver; and the text in this place may be translated, And seven priests shall bear before the ark the seven jubilee trumpets, for they appear to have been the same kind as those used on the jubilee.

Seven times - The time was thus lengthened out that the besiegers and the besieged might be the more deeply impressed with that supernatural power by which alone the walls fell.