Statement: bards using music to cast spells is stupid.
Query: how does one make bards using music to cast spells cool?
Answer: creative cursing.
What I'm thinking of is a mixture of the classic witchery (the curse works because you believe it works, because you know that someone cursed you and you're superstitious and your brain makes connections that aren't really there), and a single episode from Terry Goodkind's writing legacy about how a magic-using artist is more dangerous than anyone else - because an artist's spell doesn't depend on words. It is powered by visuals. Hiding your true name doesn't protect against it.
So: art imbued with magic can become a curse. How does it work with poetry?
Here's what you need.
1. A bard. It can be someone born with a magical ability, or someone who met a Fairy Queen and she liked him and had a one-night stand with him and seven years later he showed up and was changed forever, or someone who drank some godly mead from a magic cauldron, you can write your own d20 table of the source of bardic power if you like. Point is, there's someone who can put the magic into the rhyme and tune rather than weird gestures and mixing bat guano with pure white sand.
2. A target. Here's a retelling of an old Irish legend about Seanchan the Bard, a highly unpleasant individual. Feeling offended by a cat, he claims: "I shall satirize the tribe of the cats, and their chief lord, Irusan, son of Arusan. For I know where he lives with his wife Spit-Fire, and his daughter Sharp-Tooth, with her brothers, the Purrer and the Growler". For his offensive song to work, the bard needs to know his target well enough. It must be written in a tongue that the target understands.
3. After the bard composes and sings the song, it will magically find its way to the target (unless the target is right there next to the bard - Seanchan killed ten mice that had the misforture of hearing his ballad about them). It'll be catchy enough for people to sing and repeat it, even though they would hardly remember where they heard it first.
4. As soon as the target hears the song, bam - the curse activates. Seanchan's song led to the death of King of Cats.
5. Depending on the target and the curse, the target might have enough time and opportunity to kill the bard. The target will know who cast the curse-song, it's part of the magic. The King of Cats might've bitten Seanchan's head off if he wasn't convinced by his daughter to bring the bard alive to their cave so that they'd all take turns torturing him.
It feels like it's a mechanic best reserved for NPCs. How can it be useful in a game? Either there's an evil bard who's making a curse against someone, and you the characters must find a way to stop the bard, or there's an evil king or something and there's a bard who wants to kill the evil king with a magic song, and you the characters must provide bard with the information about the evil king for the curse to become possible. Or you hire a bard to hit your enemy with a curse-song, why not.
Don't know when, or if, I'll get to test it.