While you are rehearsing marching, your students should sing their parts. They should sing on pitch and in tune. The benefits are numerous.

If your kids can sing their parts, they will know how they should sound. 

If they can sing their parts, and blend within their section, they will play more in tune with each other.

If they sing their parts while they march new drill, it will be easier to integrate playing and marching.

If they sing their parts and project their voices, they'll use more air in their horns.

If they sing their parts and listen across the ensemble for harmony and unity, they'll do it while they play.

It's a good thing for marching bands to sing.  It's also a good thing for concert bands to sing.