The initial movement for a charge with everybody (hopefully) on the same sheet of music is the benefit of a tried chain of command, backed up by the trumpet. Orders would be given from the commander down the chain-aides going in all directions to find the appropriate commanders-and trumpet calls from the commanders to the men would be echoed through the commands. Sounds simplistic, but effective. Both drum and trumpet were the commanders' radios for that period and their sounds could not only travel quickly through the units, but it could be heard above the mess and noise.
I sure you have read Parquin, but there is some of this from his perspective as a trooper and junior officer. De Brack might also be useful in this context.
Murat's great talent was the ability to animate and move forward great masses of men and horses. One memoir states that he never came back from a charge without blood on his saber. Another account has him riding through the Prussians at Jena with noting but a 'light whip' in his hand. The Archduke Charles maintained that the French cavalry were superior to everybody else because when the trumpets sounded, they sunk spur and went at the enemy with everything they had.