Doug, Just to postulate a theory, most of the boys that were captured during the Battle of Sharpsburg were captured after they were withdrawing back to their original position after making a mile long charge driving the Union flank that was attacking the Bloody Lane. (The Charge that Saved Lee's Army)

One of the statements made by many of the Arkansas Boys was that they overran many Union soldier, who surrendered and were left in the 3rd Arkansas Boys rear as they continued the charge. I believe the 3rd Arkansas reached the Mumma Farm before they stopped and fought until they ran out of ammunition. Some of the boys called this the "Battle of the beehives" because the beehives on that farmplace were disturbed by an artillery shell and all the buzzing around their heads wasn't from yankee bullets. In fact some said that the bees were worst than the bullets.

Anyway once their ammunition ran out their flanks gave way and the withdrawal became disorganized as the 3rd withdrew back over the same ground they had covered. The yankee's who had previously surrender would jump back up and shoot at, or capture the confederates as they passed.

Now, with this in mind we look at what was the "Band" doing during this time? A true band at that time would have been following behind the main battleline to act as couriers, or to relay commands by drum, or bugle, Maybe the whole band would play to rouse the troops, or act as litterbarrer. So when the battlelines disintergrated in this disorganized withdrawal these boys may well have been left behind in somewhat of a group and captured by those Yankee's who had previously surrendered. If I were a Yankee soldier I would sure rather "capture" a Confederate Soldier who was carring a drum, or a bugle, maybe fiddle, rather that try to captured one that still had his musket and bayonet.

This is just my evaluation of the situation. Someone else may have a better answer.