The trumpet is made use of in battles, and that variously; it is used to give soldiers notice to march on against the enemy, and also to sound a retreat: if there were not a distinction in the one sound, and in the other, how should the soldier know when to go forward, and when to come back, by the sound of it? To instruct them what to do, the trumpet must not only sound, but sound intelligibly to those that hear it, which it could not, if there were no distinction in the sound.
Related Commentaries of 1 Corinthians 14:8
1 Corinthians 14:8
8 For if the trumpet give an uncertain sound, who shall prepare himself to the battle?