I can only assume the drummer is simply playing a roll because these units are broken into companies. When regiments broke into smaller units, each company or what not would only take 1 or 2 drummers. The only time you would encounter more interesting music (mulitiple drummers, fifes, etc.) would be on parade, or a full advance; situations where the regiment would be acting as one. All a single drummer would do would be give a beat or roll for marching, and give drum taps to present arms, fire, and other commands.