I've only played two types of bards;
One was a nobleman, an aristocrat who had been expensively tutored in fencing and swordplay, diplomacy, strategy and even tutored by arcane scholars until he had a smattering of magical power. He used oratory to inspire people, and provided both tactical and strategic advice from his studies of the fantasy-world-equivalent of The Art of War. The only mechanical difference between him and another bard was that he prepared spells from a collection of pages, bound into a kinda/sorta poor man's spellbook, and had less spells per day, being a prepared caster, not a spontaneous caster.
The second and third were both priests, by trade, regardless of the class written at the top of their sheet, inspiring others with religious aphorisms (enhanced by divine fervor) and gaining their spells from their religious studies.
None of the 'bards' I've played had any skill with music, song or sonic-spells (although one of the priests enjoyed a good hymn from time to time, he was no singer).