Charles H. Spurgeon on Psalms 89:15

Blessed is the people that know the joyful sound —viz., of the trumpets sounded in token of joy at the great festivals, and chiefly on the first day of the seventh month, the feast of trumpets (Le 23:24), and on extraordinary occasions, especially after the yearly atonement, on the day of jubilee, the tenth day of the seventh month of the fiftieth year, proclaiming liberty to bondmen, and restoration of their inheritance to them that had forfeited it (Le 25:8-10). As the jubilee joy did not come till after the atonement, so no Gospel joy and liberty are ours till first we know Christ as our atonement. "In the day of the people's gladness" they blew the trumpets over their sacrifices, "that they might be to them for a memorial before God" (Nu 10:10). David and Israel brought up the ark of the Lord to Zion "with shouting, and with the sound of the trumpet" (2Sa 6:15). In Nu 23:21, Balaam makes it the distinguishing glory of Israel, "The Lord his God is with him, and the shout of a king is among them", (Compare Ps 98:6 27:6 margin) —A.R. Fausset.