And their trumpets, i.e. the trumpets belonging to the whole army, even to those who were gone away, which he retained for the use here following. See Judges 7:16.
Related Commentaries of Judges 7:8
Judges 7:8
8 So the people took victuals in their hand, and their trumpets: and he sent all the rest of Israel every man unto his tent, and retained those three hundred men: and the host of Midian was beneath him in the valley.