The Drums and Colours (sometimes referred to as the 'Head Quarters' of the regiment) were the heart of the regiment on the battlefield, typically consisting of Ensigns (flag bearers), Drummers and Commanding Officers. As most, if not all, orders would come from this section, they would be separate (but not far) from the rest of the fighting regiment. As much as these days, the glory lies with the individual soldier, the world of Rank and File warfare relied on communicating with many people all at once... and the best way to do this ~400 years prior to the advent of radios was with sight and sound!
Ensigns
Ensigns carry the regimental flag, known as the "Standard" or colloquially as "Colours", and are the most junior officers on the field. The Colour is the honour of the regiment, not only indicating to the enemy who they're up against, but also provides footsoldiers a visual representation of where the regiment is standing. This also means that if the enemy is to be able to capture an enemy Standard, it will not only sew chaos into the ranks and file of hundreds of drunk and angry soldiers, but also essentially prevent your regiment from manoeuvring* and reforming smoothly and efficiently in the din of battle. I don't know if you've ever been to war (I definitely haven't) but it's bloody confusing. As a result, the Standard is to be protected at all costs - Within the Knot, we continue the tradition that if the regimental flag does so much as touches the floor, the ensign is to be lined up for a gauntletting.
Although now known to be apocryphal, a common tale was told of the first battle of the English Civil Wars at Edge Hill. It's said in the heat of battle, the Ensign of the King's Lifeguard was took as missing mid-battle. It was only at the end, after the dead and dying were being tallied and looted, that he was found... Well, at least his hands were found, still clinging to the staff of his Standard, through presumably sheer will and duty to the King (and/or Rigor Mortis).
Drummers
Considering that - for most people fighting in these wars - the loudest sound they'd ever heard would be thunder, it's not a foolish question to ask how a single man's shout could be heard over the roar of cannonade, pepperings of musket volleys, the screams of the dying and injured...
Easy answer is - Drum go 'Bang Bang Bang' real loud, innit.
Soldiers of the time were trained to react and respond to different drum calls, with different drum beats signalling different commands. Different beats to indicate that men are to prepare their weaponry, beats to indicate that a retreat is to be issued, beats to implore men to step over their fallen compatriots and hold the line. Without the drummers, communication grinds to a halt on the 17thC battlefield. Rest assured, we don't particularly need to do this within our reenactments, but I do find it a fascinating feature that I'd never considered before.
But as for the drummers themselves, they were renowned for being some of the smarter and more experienced soldiers within our ranks. Intelligent enough to keep an eye on the enemy numbers during parlays, smart enough to note them down for later use, wise enough to recognise when a battle may be lost, and cunning enough to know that if you keep your jacket's lining in another colour, people wont necessarily realise you're supposed to be on the other side of the battlefield - and subsequently wont question why you're running back and forth gaining as much information on the army's formations, losses and strengths. This is likely where the term Turncoat comes from!
Another tale from the Civil Wars featuring one drummer being returned to his frontlines during a parlay... The drummer in question had been caught gathering information and was returned inside his own drum as it was rolled back down the hill to the enemy. He wasn't in one piece, either.